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