Workmate in company left union (9)
I believe the answer is:
colleague
'workmate' is the definition.
(I've seen this before)
'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 "Fellow-worker" , "Partner" , "Partner's" , "Business co-worker" , "Fellow worker" .)