Most superior upper garment (3)
I believe the answer is:
top
'upper garment' is the definition.
'most superior' is the wordplay.
'most' means to remove the last letter.
'superior' becomes 'tops' ('tops' can be similar in meaning to 'superior').
'tops' with its final letter taken away is 'TOP'.
(Other definitions for top that I've seen before include "Peak; beat" , "premier" , "Children's toy" , "The circus tent can be a big one" , "Uppermost part" .)