The most common and most likely answer to this crossword clue is the 5 letter word BLOAT
B
L
O
A
T
More clues leading to the result BLOAT
Buddy explains!

The straight part
We know that "swell" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swell = bloat.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 BLOAT mean?
BLOAT noun- Distention of the abdomen from death.
- Pathological overdistention of rumen with gas in a ruminant.
- Wasteful use of space or other resources.
- A worthless, dissipated fellow.
- To cause to become distended.
- (veterinary medicine) to get an overdistended rumen, talking of a ruminant.
- To fill soft substance with gas, water, etc.; to cause to swell.
- To become distended; to swell up.
- To fill with vanity or conceit.
- To preserve by slightly salting and lightly smoking.
- Bloated.