Colleague having tea with firm first (10)
I believe the answer is:
stablemate
'colleague' is the definition.
(stablemate can informally mean a colleague)
'tea with firm first' is the wordplay.
'tea' becomes 'mate' (South American hot drink).
'with firm' becomes 'stable' (similar in meaning. I am not sure about the 'with' bit.).
'first' is a charade indicator (letters next to each other).
'mate' put after 'stable' is 'STABLEMATE'.
'having' is the link.
(Other definitions for stablemate that I've seen before include "Partner" , "One from the same organisation or background" , "person in same outfit" , "colleague in same organization" , "One from the same organisation as another" .)