The most common and most likely answer to this crossword clue is the 7 letter word MUDFLAT
I've seen this crossword clue in The Times
M
U
D
F
L
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T
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The straight part
We know that "land near estuary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case land near estuary = mudflat.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 MUDFLAT mean?
MUDFLAT noun- A muddy expanse of flat land, especially such land as a river bed exposed at low tide.