A professional job mostly fitting (7)
I believe the answer is:
apropos
'fitting' is the definition.
(I've seen this before)
'a professional job mostly' is the wordplay.
'professional' becomes 'pro' (abbreviation).
'job' becomes 'post' (post is a kind of job).
'mostly' means to remove the last letter (most but not all of the word).
'post' with its final letter removed is 'pos'.
'a'+'pro'+'pos'='APROPOS'
(Other definitions for apropos that I've seen before include "To the point about poor spa" , "by the way" , "fitting" , "To the point, relevant" , "Poor sap (anag) - to the purpose" .)