Burning a final act in play (9)
I believe the answer is:
fanatical
'burning' is the definition.
(thesaurus)
'a final act in play' is the wordplay.
'in play' is an anagram indicator (letters play around).
'a'+'final'+'act'='afinalact'
'afinalact' is an anagram of 'FANATICAL'.
(Other definitions for fanatical that I've seen before include "Over-zealous" , "Excessively zealous" , "Crazily keen" , "Obsessive" , "Mad" .)