One in a suit apparently wearing a vest (9)
I believe the answer is:
singleton
I believe this is a double definition.
'one in a suit' is the first definition.
(I've seen this before)
'wearing a vest' is the second definition.
(singlet on - type of vest)
'apparently' is the link.
(Other definitions for singleton that I've seen before include "lonely heart, perhaps" , "could be ace on its own?" , "Bachelor maybe" , "just one thing" , "In bridge, a holding of one card only in a suit" .)