What a hound might do for a horse (3)
I believe the answer is:
bay
'horse' is the definition.
(bay is a kind of horse)
'what a hound might do for a' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
The remaining letters 'by' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bay that I've seen before include "Inlet, cove" , "Part of sea" , "Inlet; shrub" , "Botany or Biscay, for example" , "Curved inlet; tree" .)