The most common and most likely answer to this crossword clue is the 5 letter word FATTY
I've seen this crossword clue in The Guardian
F
A
T
T
Y
Buddy explains!
The straight part
We know that "Plump person" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plump person = fatty.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 FATTY mean?
FATTY adjective- Containing, composed of, or consisting of fat.
- Like fat; greasy.
- Literally or figuratively large.
- An obese person.
- A large marijuana cigar; a blunt.