Billy who once rocked animal product on seat of Harley (4)
I believe the answer is:
fury
'billy who once rocked animal product' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'on seat of harley' is the wordplay.
'on' is a charade indicator (letters next to each other).
'seat' indicates one should take the final letters (I've seen 'seats' mean this).
'harley' becomes 'ury' (I am not sure about this - if you are sure you should believe this answer much more).
The final letter of 'of' is 'f'.
'f' next to 'ury' is 'FURY'.
Can you help me to learn more?
(Other definitions for fury that I've seen before include "Wild rage" , "Violent rage" , "Violent anger - mythological avenger" , "'Hell hath no . . . . like a woman scorned" , "Vengeful deity" .)