Bishop sat on by copper? Little beast! (3)
I believe the answer is:
cub
'little beast' is the definition.
(I know that cub is a type of young mammal)
'bishop sat on by copper?' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'sat on by' means one lot of letters go next to another.
'copper?' becomes 'cu' (Cu is the chemical symbol for copper).
'b' put after 'cu' is 'CUB'.
(Other definitions for cub that I've seen before include "A baby fox or bear" , "Inexperienced reporter" , "Young creature" , "cheeky youngster" , "Baby animal, bear say" .)