Box of ship's timber? (4)
I believe the answer is:
spar
This clue might be a double definition.
'box' is the first definition.
(spar can mean to box)
'ship's timber?' is the second definition.
'spar' can be an answer for 'timber?' (thesaurus). I am not sure about the 'ship's' bit.
'of' is the link.
Can you help me to learn more?
(Other definitions for spar that I've seen before include "Fight - something probably rigged?" , "Box; mineral; pole" , "Crystalline mineral" , "Box - girder" , "Pole used in rigging - fight" .)