Unwanted washing-line? (8)

I believe the answer is:

tidemark

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'washing-line?' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for tidemark that I've seen before include "Indication of filthy washer" , "coastline?" , "Line on shore" , "Dirt around bath" , "Dirty line in a bath" .)

I've seen this clue in The Sun.

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