Rustic boy resolved mystery (9)
I believe the answer is:
obscurity
'mystery' is the definition.
(thesaurus)
'rustic boy resolved' is the wordplay.
'resolved' is an anagram indicator.
'rustic'+'boy'='rusticboy'
'rusticboy' anagrammed gives 'OBSCURITY'.
(Other definitions for obscurity that I've seen before include "Vagueness" , "Gloominess" , "Darkness" , "Absence of fame" , "Lack of clarity" .)