Leopard one breathless catching horse? (7)
I believe the answer is:
panther
'leopard' is the definition.
(panther is a kind of leopard)
'one breathless catching horse?' is the wordplay.
'one breathless' becomes 'panter' (I can't explain this - if you can you should give a lot more credence to this answer).
'catching' means one lot of letters goes inside another.
'horse?' becomes 'h' (synonyms).
'panter' going around 'h' is 'PANTHER'.
Can you help me to learn more?
(Other definitions for panther that I've seen before include "Bagheera?" , "big pussycat" , "Then rap the large black cat" , "Black leopard" , "animal!" .)