Terror that wakes you up (5)
I believe the answer is:
alarm
'terror' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for alarm that I've seen before include "Scare with warning device" , "Agitate" , "Great concern" , "Safety bell or alert signal" , "Worried perhaps if one rings" .)