Polo mount (4)
I believe the answer is:
pony
'polo mount' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pony that I've seen before include "staked 25" , "Small horse - GBP25" , "Small breed of horse" , "Sum of 25 (Informal)" , "Animal; PS25" .)