A child among children, though mature (8)
I believe the answer is:
seasoned
'mature' is the definition.
(thesaurus)
'a child among children' is the wordplay.
'child' becomes 'son' (son is a kind of child**).
'among' means one lot of letters goes inside another.
'children' becomes 'seed' (I've seen this before).
'a'+'son'='ason'
'ason' put into 'seed' is 'SEASONED'.
'though' is the link.
(Other definitions for seasoned that I've seen before include "Gave experience to" , "Salted and peppered" , "(Of wood) dried out for use as timber" , "Veteran - given added flavour" , "Flavoured or hardened" .)