Crossword Genius

An irritability that is found trying? (9)

Ross

I believe the answer is:

testiness

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'an irritability' is the definition.
(testiness is a kind of irritability)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

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

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