Tree having a mast on? (5)

I believe the answer is:

alder

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'tree' is the definition.
(alder is a kind of tree)

I don't understand the remainder of the clue.

(Other definitions for alder that I've seen before include "Tree with catkins" , "New Forest resident" , "Forward movement" , "Tree whose bark is used in tanning" , "Shrub or tree with cone-like fruits" .)

I've seen this clue in the Evening Standard.

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