Crossword Genius

One in irrational panic or con­cealing the onset of anxiety (9)

Ross

I believe the answer is:

paranoiac

'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I cannot understand how the rest of the clue works.

(Other definitions for paranoiac that I've seen before include "Delusional" , "behaving irrationally" , "Sufferer from desusions of grandeur or persecution" , "Person suffering from delusions of persecution" .)

I've seen this clue in The Guardian.
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