Ship's mast (4)
I believe the answer is:
spar
'ship's mast' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spar that I've seen before include "Crystal - box" , "Raps with strong pole" , "Pole to support sails, rigging etc" , "Box without punching hard" , "Mineral; box; wing beam" .)