Firm butt's natural (8)
I believe the answer is:
stubborn
'firm' is the definition.
(thesaurus)
'butt's natural' is the wordplay.
'butt' becomes 'stub' ('stub' can be a synonym of 'butt').
'natural' becomes 'born' ('born' can be similar in meaning to 'natural').
'stub'+'born'='STUBBORN'
(Other definitions for stubborn that I've seen before include "Wilful" , "Obstinate, mulish" , "Difficult to move" , "Showing dogged determination" , "Unwilling to yield or grant a point" .)