The most common and most likely answer to this crossword clue is the 6 letter word POOPED
I've seen this crossword clue in The Guardian
P
O
O
P
E
D
Buddy explains!

The straight part
We know that "really tired" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case really tired = pooped.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 POOPED mean?
POOPED verb- To make a short blast on a horn
- To break wind.
- To defecate.
- To tire, exhaust. Often used with out.
- To break seawater with the poop of a vessel, especially the poop deck.
- To embark a ship over the stern.
- Tired; exhausted.
- Fitted or furnished with a poop.
- Having had a wave come over the stern from abaft.