The most common and most likely answer to this crossword clue is the 3 plus 3 letter word ALL OUT
I've seen this crossword clue in The Guardian
A
L
L
O
U
T
Buddy explains!

The straight part
We know that "Completely exhausted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case completely exhausted = all out.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 ALL OUT mean?
ALL OUT adjective- Using every available means
- Covering all aspects without exception
- In a manner in which one uses every available means, sparing no effort.