Box for a spare part (4)
I believe the answer is:
spar
This could be a double definition.
'box' is the first definition.
(sparring is a kind of boxing)
'a spare part' is the second definition.
'spar' can be an answer for 'part' (I have seen 'Part of rigging' mean 'spar' so perhaps 'part' could also mean 'spar'). I am not certain of the 'a spare' bit.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for spar that I've seen before include "Fight (in training) - pole (on boat)" , "Raps with large beam" , "Box; various minerals" , "Shadow-box" , "Mineral; box; wing beam" .)