The only one in a suit wearing vest? (9)
I believe the answer is:
singleton
'wearing 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?" , "Card that is only one held in its suit" , "Lone example" , "Eg lonely heart" , "just one thing" .)