Becomes the winner, reportedly, in children's game (7)
I believe the answer is:
conkers
'children's game' is the definition.
(game played with nuts of a horse chestnut tree)
'becomes the winner reportedly' is the wordplay.
'becomes the winner' becomes 'conquers' (conquer can mean to win).
'reportedly' shows a homophone (sound like).
'conquers' is a homophone of 'CONKERS'.
'in' is the link.
(Other definitions for conkers that I've seen before include "Schoolchildren's game" , "Game using horse chestnuts" , "Children's game" , "playground game" , "game with strikers" .)