The most common and most likely answer to this crossword clue is the 9 letter word FATIGUING
I've seen this crossword clue in The Guardian
F
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Buddy explains!

The straight part
We know that "Wearisome" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wearisome = fatiguing.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 FATIGUING mean?
FATIGUING verb- To tire or make weary by physical or mental exertion
- To wilt a salad by dressing or tossing it
- To lose so much strength or energy that one becomes tired, weary, feeble or exhausted
- (of a material specimen) to undergo the process of fatigue; to fail as a result of fatigue.