Firm: it's averse to change (9)
I believe the answer is:
assertive
'firm' is the definition.
(thesaurus)
'it's averse to change' is the wordplay.
'to change' is an anagram indicator.
'its'+'averse'='itsaverse'
'itsaverse' is an anagram of 'ASSERTIVE'.
(Other definitions for assertive that I've seen before include "Forcefully confident" , "Expressing opinions in a firm and confident manner" , "Confident, decisive" , "Confident or pushy" , "Confident, self-assured" .)