The most common and most likely answer to this crossword clue is the 9 letter word HARROWING
I've seen this crossword clue in The Daily Telegraph
H
A
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More clues leading to the result HARROWING
Buddy explains!

The straight part
We know that "Agonising" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case agonising = harrowing.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 HARROWING mean?
HARROWING verb- To drag a harrow over; to break up with a harrow.
- To traumatize or disturb; to frighten or torment.
- To break or tear, as if with a harrow; to wound; to lacerate; to torment or distress; to vex.
- The process of breaking up earth with a harrow.
- Suffering, torment.
- Christ's triumphal descent into Hell.
- Causing pain or distress.