Some chap on yonder horse (4)
I believe the answer is:
pony
'horse' is the definition.
(pony is a kind of horse)
'some chap on yonder' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'PONY' is hidden in the letters of 'chap on yonder'.
(Other definitions for pony that I've seen before include "Small horse - GBP25" , "(girl's pet, perhaps)" , "staked 25" , "Arab possibly" , "Horse (worth GBP25?)" .)