The most common and most likely answer to this crossword clue is the 8 letter word FLESHIER
I've seen this crossword clue in Independent
F
L
E
S
H
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R
More clues leading to the result FLESHIER
Buddy explains!

The straight part
We know that "more stout" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case more stout = fleshier.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 FLESHIER mean?
FLESHIER adjective- Of, related to, or resembling flesh.
- (of a person) Having considerable flesh; plump.