Inferior, firm rump (6)
I believe the answer is:
coarse
'inferior' is the definition.
('coarse' can be similar in meaning to 'inferior')
'firm rump' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'rump' becomes 'arse' ('arse' can be a synonym of 'rump').
'co'+'arse'='COARSE'
(Other definitions for coarse that I've seen before include "Vulgar; not fine" , "Rough to the touch or lacking refinement" , "Rude, base" , "Rought or crude" , "Rough or rude" .)