You do not need a separate mesh for that. It can be part of the body and it's perfectly conformed.
In Blender select only vertices for t-shirt, then in materials click "+", then "New" and then "Assign".
Now you'd have a separate material for it, which you may customize in iClone.

I have duplicated the faces of the body and fattened them.
Export as FBX and import into iClone
When in Iclone I don't see the separate mesh. ( so i can add material to this separately ) 
If I import into 3dexchange I see the duplicated part but cannot make it work there.
Is there something I need to do in Blender ?  

You could easily do that in Blender. Select appropriate body mesh faces, replicate, fatten/scale it, assign a difference material and bring it to iClone through 3DX.
But again, you cannot that it in CC nor in iClone.

Thanks for the info.
Since I can't get the character ( a fat humanoid elephant ) into CC3 is there a way in iclone to get clothes to conform.
I see I can import a t-shirt as obj but this is simply a prop.
Any workflows I can use ?
I have blender installed as well. (beginner ).
I though I could model a replicate body shape in CC3 apply the t-shirt and conform but this would not work on the elephant in Iclone.
Any ideas appreciated.

Character Creator 3 only works with CC type characters.

-------------------------------------------------------------------
liebe Grüße vidi

meet us here

As already mentioned in this thread by Peter(RL), only CC characters can be used in Character Creator. Characters created using other software can only be converted to Non Standard characters in 3DXchange for use in iClone, they can not be sent from iClone to CC however.

Gerry

---

System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.

I have a 3d boned model in blender.
In 3d exchange how do I get this nto CC3 please.
I can convert to non standard ans apply to Iclone but there is no edit in CC3 option, only edit in 3d exchange
Cheers

FYI, CC 4 will have expanded import options. 

---

OK I get it thank you.