Firm butt delivered (8)
I believe the answer is:
stubborn
'firm' is the definition.
(thesaurus)
'butt delivered' is the wordplay.
'butt' becomes 'stub' (synonyms).
'delivered' becomes 'born' (as in a baby being delivered).
'stub'+'born'='STUBBORN'
(Other definitions for stubborn that I've seen before include "Tenaciously unwilling to give in" , "Obdurate and unyielding (8)" , "Rigid" , "Difficult to move" , "Obstinate, mulish" .)