Vest being worn by one (9)
I believe the answer is:
singleton
'one' is the definition.
(singleton is a kind of one)
'vest being worn' is the wordplay.
'vest' becomes 'singlet' (synonyms).
'being worn' becomes 'on' (as in clothes having clothes on).
'singlet'+'on'='SINGLETON'
'by' is the link.
(Other definitions for singleton that I've seen before include "In bridge, a holding of one card only in a suit" , "spinster or bachelor?" , "could be ace on its own?" , "One alone" , "person not in a long-term relationship?" .)