Be suspicious when alarm's let off (5,1,3)
I believe the answer is:
smell a rat
'be suspicious' is the definition.
(I've seen this before)
'alarm's let off' is the wordplay.
'off' indicates an anagram.
'alarms'+'let'='alarmslet'
'alarmslet' with letters rearranged gives 'SMELL A RAT'.
'when' acts as a link.
(Other definitions for smell a rat that I've seen before include "Suspect trickery (colloq.)" , "Sense something suspicious" , "Suspect something wrong" , "Detect something suspicious" , "Become suspicious" .)