Just the one vest being worn (9)
I believe the answer is:
singleton
'just the' is the definition.
I can't judge whether this defines the answer.
'one vest being worn' is the wordplay.
'one vest' becomes 'singlet' (synonyms).
'being worn' becomes 'on' (as in having clothes on).
'singlet'+'on'='SINGLETON'
Can you help me to learn more?
(Other definitions for singleton that I've seen before include "could be ace on its own?" , "one without a mate?" , "in club, maybe, all alone" , "lonely heart, perhaps" , "One of a kind" .)