Black cat's spring (6)
I believe the answer is:
bounce
'spring' is the definition.
(I know that spring can be written as bounce)
'black cat's' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'cat' becomes 'ounce' (ounce can mean a snow leopard).
'b'+'ounce'='BOUNCE'
(Other definitions for bounce that I've seen before include "Springiness" , "Rebound (like a cheque)" , "Recoil like rubber ball" , "Spring back after impact" , "Spring back; resilience" .)