Try in letter to express irritability (9)
I believe the answer is:
testiness
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" .)