The most common and most likely answer to this crossword clue is the 6 letter word OEDEMA
I've seen this crossword clue in The Guardian
O
E
D
E
M
A
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The straight part
We know that "Swelling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swelling = oedema.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 OEDEMA mean?
OEDEMA noun- An excessive accumulation of serum in tissue spaces or a body cavity
- A similar swelling in plants caused by excessive accumulation of water