Non-professional in a mature turn-out (7)
I believe the answer is:
amateur
'non-professional' is the definition.
(I've seen this before)
'a mature turn-out' is the wordplay.
'turn-out' is an anagram indicator.
'mature' with letters rearranged gives 'mateur'.
'a'+'mateur'='AMATEUR'
'in' acts as a link.
(Other definitions for amateur that I've seen before include "One is not professional" , "Fancier" , "he won't get paid" , "Non-pro" , "Non professional, eight across" .)