The most common and most likely answer to this crossword clue is the 4 letter word VLEI
V
L
E
I
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The straight part
We know that "a swamp" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a swamp = vlei.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 VLEI mean?
VLEI noun- (Southern Africa) A shallow wetlands or minor lake, generally a seasonal one, or the lowland where such a wetlands appears seasonally.
- A wetland or marsh.