Sudden overpowering anxiety (5)
I believe the answer is:
panic
'sudden overpowering anxiety' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for panic that I've seen before include "Sudden feeling of terror" , "Alarm, scare" , "Frantic fear" , "Fearful confusion" , "Sudden, infectious fear" .)