As worn beneath the tree-tops? (6)

I believe the answer is:

trunks

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'as' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I don't understand the remainder of the clue.

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(Other definitions for trunks that I've seen before include "Man's swimming costume" , "Swimwear" , "luggage" , "Main stems of trees" , "Parts of trees and elephants" .)

I've seen this clue in the Evening Standard.

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