Small horse (4)
'small horse' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pony that I've seen before include "Twenty-five pounds; small horse" , "Horse (worth PS25?)" , "Small breed of horse" , "Animal; PS25" , "staked 25" .)
(Other definitions for shetland pony that I've seen before include "Steed" , "Small breed of horse" , "Rough-coated mount" , "Small, shaggy-maned animal" .)