Contemptuous gain is included (8)
I believe the answer is:
derisive
'contemptuous' is the definition.
(I've seen this before)
'gain is included' is the wordplay.
'gain' becomes 'derive' (synonyms).
'included' indicates putting letters inside.
'derive' placed around 'is' is 'DERISIVE'.
(Other definitions for derisive that I've seen before include "'Scornful, mocking (8)'" , "Taunting" , "Expressing contempt" , "Contemptuous" .)