Associate in company left union (9)
I believe the answer is:
colleague
'associate' is the definition.
(colleague is a kind of associate)
'company left union' is the wordplay.
'company' becomes 'co' (abbreviation).
'left' becomes 'l' (common abbreviation).
'union' becomes 'league' (league is a kind of union).
'co'+'l'+'league'='COLLEAGUE'
'in' acts as a link.
(Other definitions for colleague that I've seen before include "Person with whom one works" , "Workmate" , "Partner" , "Fellow worker" , "Partner's" .)