Washing line? (8)
I believe the answer is:
tidemark
'washing line?' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'line?' is the wordplay.
I cannot really see how this works, but
'line?' could be 'mark' (line is a kind of mark) and 'mark' is found in the answer.
The remaining letters 'tide' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tidemark that I've seen before include "Grimy ring left on bath" , "Dirt around bath" , "line on beach" , "Line on shore" , "shows extent of clean-up" .)