Unplanned opus - a sonnet, rewritten (11)
I believe the answer is:
spontaneous
'unplanned' is the definition.
(thesaurus)
'opus a sonnet rewritten' is the wordplay.
'rewritten' indicates an anagram.
'opus'+'a'+'sonnet'='opusasonnet'
'opusasonnet' anagrammed gives 'SPONTANEOUS'.
(Other definitions for spontaneous that I've seen before include "Cause of Mr Krook's death" , "Unprompted" , "Impulsive, unpremeditated" , "Impromptu" .)