Leopard without spots, one breathing heavily, biting hard (7)
I believe the answer is:
panther
'leopard' is the definition.
(I know that panther is a type of leopard)
'heavily biting hard' is the wordplay.
'heavily' becomes 'panter' (I can't justify this - if you can you should give a lot more credence to this answer).
'biting' means one lot of letters goes inside another (some letters bite or swallow others).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'panter' placed around 'h' is 'PANTHER'.
'without spots one breathing' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for panther that I've seen before include "Big cat" , "See 2" , "Black leopard" , "big pussycat" , "Bagheera?" .)