An irritability that is found trying? (9)

I believe the answer is:

testiness

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'an irritability' is the definition.
(I know that testiness is a type of choler)

I can't explain the rest of the clue.

(Other definitions for testiness that I've seen before include "Irascibility" , "Irritability" .)

I've seen this clue in the Evening Standard.

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