Infant sat on large racket (5)
I believe the answer is:
babel
'racket' is the definition.
(I've seen this before)
'infant sat on large' is the wordplay.
'infant' becomes 'babe' (synonyms).
'sat on' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'large' becomes 'L' (eg in clothes sizes).
'babe'+'l'='BABEL'
(Other definitions for babel that I've seen before include "Commotion" , "Noisy confusion of voices" , "incoherent sounds" , "Biblical city with a tower" , "Ancient city" .)