The most common and most likely answer to this crossword clue is the 5 letter word SEDGE
I've seen this crossword clue in The Times
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More clues leading to the result SEDGE
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The straight part
We know that "Marsh plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marsh plant = sedge.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 SEDGE mean?
SEDGE noun- Any plant of the genus Carex, the true sedge, perennial, endogenous herbs, often growing in dense tufts in marshy places. They have triangular jointless stems, a spiked inflorescence, and long grasslike leaves which are usually rough on the margins and midrib. There are several hundred species.
- Any plant of the family Cyperaceae.
- Certain other plants resembling sedges, such as Gentiana rubricaulis and Andropogon virginicus.
- A dry fly used in fly fishing, designed to resemble a sedge or caddis fly.
- Alternative spelling of segge
- A flock of herons, cranes, or bitterns.
- (heading) Military action.
- (heading) A seat.
- A place with a toilet seat: an outhouse; a lavatory.