Notice allegedly found next to small opening in dense vegetation (7)

I believe the answer is:

seaweed

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But I can't explain it at all! Could you help me understand?

(Other definitions for seaweed that I've seen before include "Bladderwrack, for example" , "What may be cast up on shore" , "We eased (anag)" , "water plant" , "Iodine source" .)

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