The most common and most likely answer to this crossword clue is the 4 letter word SOSS
S
O
S
S
Buddy explains!
The straight part
We know that "plump" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plump = soss.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 SOSS mean?
SOSS noun- Anything dirty or muddy; a dirty puddle.
- To soil; to make dirty.
- A lazy fellow.
- A heavy fall.
- To fall suddenly into a chair or seat; to sit lazily.
- To throw in a negligent or careless manner; to toss.