Colleague is firm friend (10)
I believe the answer is:
stablemate
'colleague' is the definition.
(I've seen this before)
'firm friend' is the wordplay.
'firm' becomes 'stable' (similar in meaning).
'friend' becomes 'mate' (mate is a kind of friend).
'stable'+'mate'='STABLEMATE'
'is' acts as a link.
(Other definitions for stablemate that I've seen before include "Nag" , "Blames Tate (anag) - person from the same club" , "One from the same organisation or background" , "person in same outfit" , "One from the same organisation as another" .)