Frightened when security system is active (7)
I believe the answer is:
alarmed
'frightened' is the definition.
(alarming is a kind of frightening)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for alarmed that I've seen before include "Frightened, scared" , "Fearful" , "Given a turn" , "Anxious; disturb" , "as a door handle may be?" .)