Kind of varnish provided by company colleague (5)
I believe the answer is:
copal
'kind of varnish' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'by company colleague' is the wordplay.
'by' is a charade indicator (letters next to each other).
'company' becomes 'co' (abbreviation).
'colleague' becomes 'pal' (I can't justify this - if you can you should believe this answer much more).
'co' next to 'pal' is 'COPAL'.
'provided' acts as a link.
Can you help me to learn more?
(Other definitions for copal that I've seen before include "Aromatic tree resin" , "Resin used in varnish" , "Hard resin" .)