Suspect lets alarm go off (5,1,3)
I believe the answer is:
smell a rat
'suspect' is the definition.
(I've seen this before)
'lets alarm go off' is the wordplay.
'go off' indicates anagramming the letters.
'lets'+'alarm'='letsalarm'
'letsalarm' anagrammed gives 'SMELL A RAT'.
(Other definitions for smell a rat that I've seen before include "Suspect something's amiss" , "Suspect trickery (colloq.)" , "Suspect deception" , "Have suspicion something is afoot" , "Suspect something wrong" .)