Only one vest? Not quite (6)
I believe the answer is:
single
'only one' is the definition.
(I've seen this before)
'vest? not quite' is the wordplay.
'vest?' becomes 'singlet' ('singlet' can be a synonym of 'vest').
'not quite' means to remove the last letter.
'singlet' with its last letter taken away is 'SINGLE'.
(Other definitions for single that I've seen before include "Concentrating on only one aim" , "Unique" , "Sole" , "Individual - unmarried" , "Unaccompanied" .)