Practise boxing in the yard (4)
I believe the answer is:
spar
I believe this clue is a double definition.
'practise boxing' is the first definition.
(I've seen this before)
'yard' is the second definition.
(I've seen this before)
'in the' is the link.
(Other definitions for spar that I've seen before include "Pole to support sails, rigging etc" , "piece of wood?" , "Box in training - nautical pole" , "quarrel" , "Argue" .)