On snooker table, screw supports leg (8)
I believe the answer is:
'on snooker table screw' is the definition.
I can't tell whether this definition defines the answer.
'supports leg' is the wordplay.
'supports' becomes 'backs' (backing is a kind of supporting).
'leg' becomes 'pin' (pin is a kind of leg).
Can you help me to learn more?
(Other definitions for backspin that I've seen before include "It slows or reverses a ball's forward motion" , "Rotary movement contrary to direction of travel" , "Ball's rotation" , "What gives a ball lift" , "Pink scab (anag.)" .)