The most common and most likely answer to this crossword clue is the 5 letter word FAINT
I've seen this crossword clue in The Daily Telegraph
F
A
I
N
T
More clues leading to the result FAINT
Buddy explains!

The straight part
We know that "Pale/pass out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pale/pass out = faint.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 FAINT mean?
FAINT noun- The act of fainting, syncope.
- The state of one who has fainted; a swoon.
- (of a being) Lacking strength; weak; languid; inclined to lose consciousness
- Lacking courage, spirit, or energy; cowardly; dejected
- Barely perceptible; not bright, or loud, or sharp
- Performed, done, or acted, weakly; not exhibiting vigor, strength, or energy
- Slight; minimal.
- To lose consciousness through a lack of oxygen or nutrients to the brain, usually as a result of suddenly reduced blood flow (may be caused by emotional trauma, loss of blood or various medical conditions).
- To sink into dejection; to lose courage or spirit; to become depressed or despondent.
- To decay; to disappear; to vanish.