The most common and most likely answer to this crossword clue is the 8 letter word WELL-FED
I've seen this crossword clue in Financial Times
W
E
L
L
-
F
E
D
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 = well-fed.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 WELL-FED mean?
WELL-FED adjective- Properly or adequately fed or nourished