The most common and most likely answer to this crossword clue is the 3 letter word FEN
I've seen this crossword clue in The Daily Telegraph
F
E
N
More clues leading to the result FEN
Buddy explains!
The straight part
We know that "Swamp" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swamp = fen.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 FEN mean?
FEN noun- A type of wetland fed by ground water and runoff, containing peat below the waterline, characteristically alkaline.
- Unit of currency in China, one-hundredth of a yuan
- Fans; a plural form used by enthusiasts of science fiction, fantasy, and anime, partly from whimsy and partly to distinguish themselves from fans of sport, etc.
- A kind of mildew that grows on hops.