Trees with coats, say (4)

I believe the answer is:

firs

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'trees with' is the definition.
'firs' can be an answer for 'trees' (I've seen this before). I am not certain of the 'with' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for firs that I've seen before include "Evergreens" , "Trees" .)

I've seen this clue in The Mirror.

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