Posh lass with racket on grand horse (7)
I believe the answer is:
gelding
'horse' is the definition.
(I know this)
'posh lass with racket on grand' is the wordplay.
'posh lass' becomes 'gel' (I've seen this before).
'with' means one lot of letters go next to another.
'racket' becomes 'din' (both can mean a loud noise or disturbance).
'on' is a charade indicator (letters next to each other).
'grand' becomes 'G' (G is short for ground as in a thousand).
'gel'+'din'+'g'='GELDING'
(Another definition for gelding that I've seen is " Castrated male horse".)