A slight tangent here...
For the most part, I know it is recommended that if you are importing custom clothing into CC3 that your avatar be in the T-pose position. Now, a lot of times, I make my own clothes in external programs like Blender, and in so doing, I export a CC3 generation avatar from the Pipeline version of CC3 with the T-pose. Quite honestly, this is the best way to ensure that if you are making your own clothes, that those clothes will fit your avatar when exporting them into CC3. There is a reason I said... "A slight tangent here...." and I'll go into that in a bit, but for now....
Regardless, whether you are importing clothes from your own creation, or from an online shop, make sure you Calculate Collision after Transferring Skin Weights. You might have to Calculate Collision a couple of times to make sure that there are no bare spots on the clothing where there should be cloth. After you've Calculated Collision, use a couple of relatively (for lack of a better word) contorted pose, (for females, the best test poses I've found are "Pray" and "Enticing" and look at the clothing on your character from those positions....smooth out, sculpt where necessary... and try to make sure you don't oversculpt. Usually, if all goes well, a couple of long, broad strokes in the exposed area will clear up those bare spots. Once you have patched up those areas with some scuplting, return the avatar to T-pose ...quick method is look to the Modify panel and click on the Motion Pose>Edit Pose and then click on "default". (side note: If you've somehow accidentally screwed up your default option in the past, then just go into the Content panel on the left side, and click on Pose>Face&Body>Default or Pose>Body>0_T_Pose then go back to the Modify>Motion Pose>Edit Pose...and click on Set As Default. Now T-pose has been restored as your default pose in the Edit Pose panel. After you've done the extreme poses, there is still one more test. In the Edit Pose function on the Modify panel, select the abdomen point (the point above the waist marker on the Edit Pose diagram) and bend your character a bit forward... say, almost L shaped (look for bare patches, sculpt out, if any), and a bit backward, not quite L shaped...just enough to look slightly painful LOL (look for bare patches and correct if and where necessary). For the most part now, your clothes should be well fit and ready to animate on your iClone CC3 characters.
Ok...now, for the "A Slight Tangent Here...." portion of this post. As I stated before, I usually make my own clothes in Blender using an exported CC3 gen avatar from Character Creator 3 Pipeline. Tops/shirts are usually fairly easy to create in Blender on your CC3 mesh... just a bit of knowing what faces to highlight in Edit Mode, where to make your Knife cuts or Knife Projection cuts, etc. Sometimes however, things like pants, shorts, and gloves can be a bit problematic because of the proximity of limbs to each other. (legs proximity to each other, or fingers proximity to each other in T-pose). When importing your pants/shorts/gloves into CC3, you might have to do a lot of Lateral Partition editing if your pants or shorts are not exactly "form fitting"...if they're loose or even baggy. Gloves can be even more problematic because the fingers are straight out and inline...a flat hand.
To resolve this.... I create a pose in CC3 for my avatar where the feet stand apart (shoulder width, or slightly broader, should be good enough), and I spread the fingers out manually....not too wide, just enough to have some space between them. Do not flex the fingers and thumb...and, in fact, try to ensure that your thumb is level with your extended fingers. Once I'm satisfied with the limb and digital positioning, I will export that avatar as an .obj for use in Blender. (I saved this custom pose in CC3 custom pose folder for future imports of new clothing). In Blender, I'll create the clothes (I did a pictorial tut on this sometime back, but I think I'll be updating and making a new tut soon), and then export them as .objs to be reimported into CC3 onto my posed avatar. As I mentioned earlier, the advantage of using exported .obj avatar meshes in Blender is that your clothes will automatically be the correct scale for your avatar in Character Creator 3 when you import your custom made clothes as Accessories. Do the usual.... Transfer skin weights (with Lateral Partitioning on just to make double sure), then calculate collision. Now, your pants/shorts/gloves should fit with minimal (if any) partitioning fuss after Calculate Collision. Then do the usual tests: put your avatar in default T-pose, look it over, check for any bare spots, sculpt if necessary. Use those extreme poses I'd mentioned earlier, each time, looking over the clothes, checking for bare patches. This may look (in text) like it would take forever, but it really only takes a few minutes.
I hope this TL;DR helps. :-)
"Incompetence will always prevail so long as evil men stand by and do nothing."