Suspect something's wrong -- let alarms off (5,1,3)
I believe the answer is:
smell a rat
'suspect something's wrong' is the definition.
'let alarms off' is the wordplay.
'off' indicates an anagram.
'let'+'alarms'='letalarms'
'letalarms' is an anagram of 'SMELL A RAT'.
(Other definitions for smell a rat that I've seen before include "Become suspicious" , "Find something fishy" , "Suspect something's amiss" , "Suspect trickery (colloq.)" , "Suspect deception" .)