Opt to change upper garment (3)
I believe the answer is:
top
'upper garment' is the definition.
'opt to change' is the wordplay.
'to change' indicates an anagram.
'opt' with letters rearranged gives 'TOP'.
(Other definitions for top that I've seen before include "Children's toy" , "The circus tent can be a big one" , "Upper side" , "SECOND PART OF 10 ACROSS" , "Kill" .)