Co-worker in company left union (9)
I believe the answer is:
colleague
'co-worker' is the definition.
('colleague' can be a synonym of 'co-worker')
'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' is the link.
(Other definitions for colleague that I've seen before include "fellow member of staff" , "Business co-worker" , "Fellow worker" , "Person with whom one works" , "Partner's" .)