The most common and most likely answer to this crossword clue is the 7 letter word ESTUARY
I've seen this crossword clue in The Daily Telegraph
E
S
T
U
A
R
Y
More clues leading to the result ESTUARY
- Royal family member nearly punching you once round mouth
- English house, unfinished, close to Mersey’s mouth
- English royal family’s clipped semivowel – it’s not the Queen’s English!
- Wide mouth contorted, say? True
- Water at the mouth
- Mouthful of water
- Quaint Rye sat either side of bend in river approaching the sea
- Sure, Tay at sea exhibits it
- Form of English developing as true unknown quantity
- . True! Oddly took rigorous years around to find part of river
Buddy explains!

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 ESTUARY mean?
ESTUARY noun- Coastal water body where ocean tides and river water merge, resulting in a brackish water zone.
- An ocean inlet also fed by fresh river water.