Fellow worker in firm left association (9)
I believe the answer is:
colleague
'fellow worker' is the definition.
('colleague' can be a synonym of 'fellow worker')
'firm left association' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'left' becomes 'l' (common abbreviation).
'association' becomes 'league' (I've seen this before).
'co'+'l'+'league'='COLLEAGUE'
'in' is the link.
(Other definitions for colleague that I've seen before include "fellow member of staff" , "Partner's" , "Fellow-worker" , "Fellow worker" , "Partner" .)