The most common and most likely answer to this crossword clue is the 8 letter word TIDEMARK
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result TIDEMARK
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The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TIDEMARK mean?
TIDEMARK noun- A line (of seaweed or differently coloured sand etc) on the shore showing the level of high or low tide
- (by extension) any mark showing the limit of some past activity
- A line of scum left on a bath tub when the water is drained away