Hi,I don't know if you have read the developer document "Creating Characters For iClone" but if not I would strongly advise you to do so as it contains some vital information on the issues you have. There are also some video tutorials which are well worth following too. All of which can be found in the Developer Center.
http://developer.reallusion.com/
Basically any clothing you create for the G1 characters needs to be part of the Upper or Lower mesh. If the clothing is separate it will not animate with the character as you are experiencing now. You can't have an Upper or Lower mesh and an additional clothing mesh when exporting to iClone you should combine them into one Upper and one Lower.
Once you have created the clothing you will need to use the Skin Modifier to make sure that the mesh moves and bends correctly with the bones when the character is animated.
I hope this points you in the right direction.
Pete