"About painting", I snap unfairly, "is biased" (8)
I believe the answer is:
partisan
'is biased' is the definition.
('partisan' can be similar in meaning to 'biased')
'about painting i snap unfairly' is the wordplay.
'about' is an insertion indicator.
'painting' becomes 'art' (I've seen this before).
'unfairly' indicates an anagram (I've seen this in other clues).
'i'+'snap'='isnap'
'isnap' anagrammed gives 'pisan'.
'art' placed within 'pisan' is 'PARTISAN'.
(Other definitions for partisan that I've seen before include "Prejudiced towards one side" , "Guerrilla" , "Unquestioning supporter -- aspirant (anag)" , "pike" , "Strong party supporter" .)