The most common and most likely answer to this crossword clue is the 5 letter word BAYOU
I've seen this crossword clue in Independent
B
A
Y
O
U
More clues leading to the result BAYOU
- The solver on degree course by lake?
- Opening where French create a swampy river outlet
- Second person found on half-exposed marshy area
- Inlet in Orleans, or marshy water in New Orleans?
- Parking area over by university that’s swamped in US
- Marsh inlet opens up at first
- Solver pursuing Boatman up marshy creek
- A buoy out of place in marshy outlet
Buddy explains!

The straight part
We know that "marshy area" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marshy area = bayou.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 BAYOU mean?
BAYOU noun- A slow-moving, often stagnant creek or river.
- A swamp, a marshy (stagnant) body of water.