Couple sounding trim (4)
I believe the answer is:
pair
'couple' is the definition.
(I know that couple can be written as pair)
'sounding trim' is the wordplay.
'sounding' indicates a 'sounds like' (homophone) clue.
'trim' becomes 'pare' (synonyms).
'pare' sounds like 'PAIR'.
(Other definitions for pair that I've seen before include "Twosome" , "item" , "Twins" , "Brace, couple" , "Agreement to avoid division" .)