Crossword Genius

Calmness (or irritability, la Superman?) (6,2,5)

Ross

I believe the answer is:

nerves of steel

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'calmness' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

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(Another definition for nerves of steel that I've seen is " They show calmness under pressure".)

I've seen this clue in The Guardian.
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