Nun's a poet, so madly impulsive (11)
I believe the answer is:
spontaneous
'impulsive' is the definition.
(similar in meaning)
'nun's a poet so madly' is the wordplay.
'madly' indicates an anagram.
'nuns'+'a'+'poet'+'so'='nunsapoetso'
'nunsapoetso' is an anagram of 'SPONTANEOUS'.
(Other definitions for spontaneous that I've seen before include "Impulsive, unpremeditated" , "Impromptu" , "Cause of Mr Krook's death" , "Unprompted" .)