Extreme fright (6)
(Other definitions for panic that I've seen before include "Lose one's cool" , "Lose one's nerve" , "Frenzy" , "Lose the head, yield to fear" , "Extreme fear" .)
'extreme fright' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for terror that I've seen before include "Mischief-maker; dread" , "rogue" , "Little brat" , "Utter dread" , "Extreme or intense fear" .)