Among children, a boy is mature (8)
I believe the answer is:
seasoned
'is mature' is the definition.
(thesaurus)
'among children a boy' is the wordplay.
'among' is an insertion indicator (I've seen this in other clues).
'children' becomes 'seed' (I've seen this before).
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'a'+'son'='ason'
'seed' placed around 'ason' is 'SEASONED'.
(Other definitions for seasoned that I've seen before include "Salted and peppered" , "Added spice to" , "Mature" , "Of food, flavoured with salt, pepper etc" , "Gave experience to" .)