Firm flying out in case (9)
I believe the answer is:
tenacious
'firm' is the definition.
(thesaurus)
'flying out in case' is the wordplay.
'flying' indicates an anagram.
'out'+'in'+'case'='outincase'
'outincase' anagrammed gives 'TENACIOUS'.
(Other definitions for tenacious that I've seen before include "Holding or sticking firmly" , "Holding fast" , "Keeping a firm hold" , "Persevering" , "refusing to give up" .)