Suspect let alarms off (5,1,3)
I believe the answer is:
smell a rat
'suspect' is the definition.
(I've seen this before)
'let alarms off' is the wordplay.
'off' is an anagram indicator.
'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" , "Suspect deception" , "Have suspicion something is afoot" , "Detect something suspicious" , "Suspect something wrong" .)