More adult game about to be curbed by Etonian's mum (7)
I believe the answer is:
maturer
'more adult' is the definition.
'game about to be curbed by etonian's mum' is the wordplay.
'game' becomes 'ru' (rugby union).
'about' shows that the letters should be reversed in order.
'to be curbed by' indicates putting letters inside (I've seen this in other clues).
'etonian's mum' becomes 'mater' (Latin term for mother - Latin is taught at Eton).
'ru' written backwards gives 'ur'.
'ur' put within 'mater' is 'MATURER'.
(Other definitions for maturer that I've seen before include "It's more sensible" , "More ripe" , "It's probably wiser" , "Older and wiser" , "More grown up" .)