Crossword Genius

The irritability of the sandwich man? (10)

Ross

I believe the answer is:

crustiness

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

'the irritability' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's an association between them I don't understand?

'the sandwich man?' is the wordplay.
I cannot really understand how this works, but
'man?' could be 'rustin' and 'rustin' is present in the answer.
The remaining letters 'cess' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'of' acts as a link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.
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