Grew up enthusiastic about true criminal (7)
I believe the answer is:
matured
'grew up' is the definition.
(to grow up is to mature)
'enthusiastic about true criminal' is the wordplay.
'enthusiastic' becomes 'mad' (mad about something means enthusiastic about it).
'about' is an insertion indicator.
'criminal' is an anagram indicator.
'true' anagrammed gives 'ture'.
'mad' enclosing 'ture' is 'MATURED'.
(Other definitions for matured that I've seen before include "Mellowed" , "Ripened" , "Developed; grew up" , "Developed fully" , "Became fully ripe" .)