Piece let into a garment (5)
I believe the answer is:
inset
'piece let into a garment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for inset that I've seen before include "Extra bit of garment" , "It could be stuck between pages" , "Training during term time for teachers in state schools" , "Small diagram contained by larger one" , "lesser feature" .)