The only one in a suit wearing a vest (9)
I believe the answer is:
singleton
'wearing a vest' is the definition.
(singlet on - type of vest)
'the only one in a suit' is the wordplay.
'the only' becomes 'single' (I've seen this before).
'one in a suit' becomes 'ton' (I've seen this in other clues).
'single'+'ton'='SINGLETON'
(Other definitions for singleton that I've seen before include "could be ace on its own?" , "free spirit" , "Anything on its own" , "One object only" , "Lone example" .)