Return blows to practise boxing (4)
I believe the answer is:
spar
'practise boxing' is the definition.
(I've seen this before)
'return blows' is the wordplay.
'return' says the letters should be written in reverse.
'blows' becomes 'raps' (rap is a kind of blow).
'raps' reversed gives 'SPAR'.
'to' is the link.
(Other definitions for spar that I've seen before include "Mineral; plane's wing beam" , "Argue" , "supporting member" , "Box in training - mineral" , "Sticks" .)