The most common and most likely answer to this crossword clue is the 6 letter word LAGOON
I've seen this crossword clue in Financial Times
L
A
G
O
O
N
More clues leading to the result LAGOON
- Crazy person crossing a grand shallow lake?
- Proceed east of coastal city to find calm water
- Olga’s travelling on shallow lake
- The French journey on a shallow lake
- Body of water in the past flooding half the capital
- Get behind second in command on stretch of water
- Lake city in USA – really?
- Fall behind duck on lake
- Where young lady returning with nothing on may take a dip?
- Trail duck on small lake
Buddy explains!

The straight part
We know that "shallow lake" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shallow lake = lagoon.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 LAGOON mean?
LAGOON noun- A shallow body of water separated from deeper sea by a bar.