Suspect something left inside set alarm off (5,1,3)
I believe the answer is:
smell a rat
'suspect something' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for smell a rat that I've seen before include "Find something fishy" , "Sense something suspicious" , "Become suspicious" , "Suspect deception" , "Have suspicion something is afoot" .)