The most common and most likely answer to this crossword clue is the 7 letter word PEAKIER
I've seen this crossword clue in The Guardian
P
E
A
K
I
E
R
Buddy explains!

The straight part
We know that "less well" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case less well = peakier.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 PEAKIER mean?
PEAKIER adjective- Sickly; peaked.
- Characterised by peaks.