The most common and most likely answer to this crossword clue is the 5 letter word FATSO
I've seen this crossword clue in Independent
F
A
T
S
O
More clues leading to the result FATSO
Buddy explains!

The straight part
We know that "One well-upholstered" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one well-upholstered = fatso.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 FATSO mean?
FATSO noun- Someone who is overweight.