A beam of boxwood? (4)

I believe the answer is:

spar

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'a beam' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for spar that I've seen before include "dispute" , "Engage in argument" , "Pole used to support sails or rigging" , "Go a few rounds with the ship's pole" , "Box (gently) - fight (verbally)" .)

I've seen this clue in the Evening Standard.

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