Crossword Genius

Try in letter to express irritability (9)

Ross

I believe the answer is:

testiness

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

'try in letter' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

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

'to express' is the link.

Can you help me to learn more?

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

I've seen this clue in the Sydney Morning Herald.
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