Box for a piece of the rigging (4)
I believe the answer is:
spar
'rigging' is the definition.
(I know that spar is a type of rigging)
'box for a piece' is the wordplay.
'box for' becomes 'spa' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a piece' becomes 'r' (abbreviation for Rook).
'spa'+'r'='SPAR'
'of the' acts as a link.
Can you help me to learn more?
(Other definitions for spar that I've seen before include "Box; mineral; pole" , "Pole used in rigging - fight" , "Close friend" , "Wooden pole -- box" , "Box in training - nautical pole" .)