Spies dry, gnarled trees by river with very frozen water (6,7)

I believe the answer is:

secret service

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

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

(Other definitions for secret service that I've seen before include "Hush-hush organisation" , "MI5 or MI6 (informal)" , "Government department involved in espionage" , "Spying agency" , "Spies' department" .)

I've seen this clue in The Guardian.

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