The most common and most likely answer to this crossword clue is the 8 letter word WEARIEST
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The straight part
We know that "tired in the extreme" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tired in the extreme = weariest.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 WEARIEST mean?
WEARIEST adjective- Having the strength exhausted by toil or exertion; tired; fatigued.
- Having one's patience, relish, or contentment exhausted; tired; sick.
- Expressive of fatigue.
- Causing weariness; tiresome.