Rugby player's leg with which ball may be struck (8)
I believe the answer is:
backspin
'with which ball may be struck' is the definition.
(a ball may be hit with backspin)
'rugby player's leg' is the wordplay.
'rugby player's' becomes 'backs'.
'leg' becomes 'pin' (pin is a kind of leg).
'backs'+'pin'='BACKSPIN'
(Other definitions for backspin that I've seen before include "It slows or reverses a ball's forward motion" , "Lifting rotation of ball" , "Rotary movement contrary to direction of travel" , "What gives a ball lift" , "Ball's rotation" .)