A mature sort of bungler (7)
I believe the answer is:
amateur
'bungler' is the definition.
(thesaurus)
'a mature sort' is the wordplay.
'sort' is an anagram indicator.
'mature' with letters rearranged gives 'mateur'.
'a'+'mateur'='AMATEUR'
'of' acts as a link.
(Other definitions for amateur that I've seen before include "Inept, non-professional" , "11 [UNTRAINED]" , "Unpaid (sport)" , "A non-professional sportsman" , "refusing payment" .)