Bust itches -- aunt is warm (12)
I believe the answer is:
enthusiastic
'warm' is the definition.
(I've seen this before)
'bust itches aunt is' is the wordplay.
'bust' indicates anagramming the letters.
'itches'+'aunt'+'is'='itchesauntis'
'itchesauntis' with letters rearranged gives 'ENTHUSIASTIC'.
(Other definitions for enthusiastic that I've seen before include "Eager" , "Gung-ho" , "Ardent, zealous" , "hot" , "Highly keen" .)