The most common and most likely answer to this crossword clue is the 4 letter word MIRE
M
I
R
E
More clues leading to the result MIRE
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 = mire.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 MIRE mean?
MIRE noun- Deep mud; moist, spongy earth.
- An undesirable situation, a predicament.
- To cause or permit to become stuck in mud; to plunge or fix in mud.
- To sink into mud.
- To weigh down.
- To soil with mud or foul matter.
- An ant.