Practise fighting, taking knocks from behind (4)
I believe the answer is:
spar
'practise fighting' is the definition.
'knocks from behind' is the wordplay.
'knocks' becomes 'raps' ('rap' can be a synonym of 'knock').
'from behind' says the letters should be written backwards.
'raps' in reverse letter order is 'SPAR'.
'taking' is the link.
(Other definitions for spar that I've seen before include "Box - mineral - rod" , "Mineral; strong pole" , "Box (gently) - fight (verbally)" , "Box gently; main beam of aircraft wing" , "Box; mineral; pole" .)