Horse-riding brawl (5-2)
I believe the answer is:
punch-up
'brawl' is the definition.
(I've seen this before)
'horse-riding' is the wordplay.
'horse' becomes 'punch' (type of horse).
'riding' becomes 'up' (I've seen this in other clues).
'punch'+'up'='PUNCH UP'
(Other definitions for punch-up that I've seen before include "Brawl (colloq.)" , "An informal fight, with vertical blows?" , "Fist fight" , "Fisticuffs incident" , "An informal brawl, a fight with fists" .)