Clue: One in irrational panic or con­cealing the onset of anxiety (9)
Answer: PARANOIAC
Explanation: One in irrational panic or con­cealing the onset of anxiety (9)
An envelope ('concealing') of A ('the onset of **A**nxiety') in PARANOIC, an envelope ('in') of A ('one') in PARNOIC, an anagram ('irrational') of 'panic or', with an &lit (roughly) definition. Take your choice of which A is which.