The most common and most likely answer to this crossword clue is the 5 letter word PALER
I've seen this crossword clue in The Daily Telegraph
P
A
L
E
R
More clues leading to the result PALER
Buddy explains!

The straight part
We know that "looking less well" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case looking less well = paler.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 PALER mean?
PALER adjective- Light in color.
- (of human skin) Having a pallor (a light color, especially due to sickness, shock, fright etc.).
- Feeble, faint.