Fellow worker in company left union (9)
I believe the answer is:
colleague
'fellow worker' is the definition.
('colleague' can be a synonym of 'fellow 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' acts as a link.
(Other definitions for colleague that I've seen before include "Associate" , "Fellow-worker" , "Business co-worker" , "Partner" , "China, perhaps" .)