Firm buttocks of low quality (6)
I believe the answer is:
coarse
'of low quality' is the definition.
'firm buttocks' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'buttocks' becomes 'arse' ('arse' can be a synonym of 'buttocks').
'co'+'arse'='COARSE'
(Other definitions for coarse that I've seen before include "lacking in taste" , "Rough in texture, or vulgar" , "needing refinement" , "Rough or rude" , "Unpolished -- ribald" .)