Mount, possibly on a racehorse (4)
I believe the answer is:
pony
'racehorse' is the definition.
(pony is a kind of racehorse)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for pony that I've seen before include "Small horse - GBP25" , "(girl's pet, perhaps)" , "Horse (worth PS25?)" , "Small horse - PS25" , "Horse (worth GBP25?)" .)