Alarm's let off? Suspect something's up (5,1,3)
I believe the answer is:
smell a rat
'suspect something's up' is the definition.
(I've seen this before)
'alarm's let off?' is the wordplay.
'off?' indicates anagramming the letters (I've seen 'off' mean this).
'alarms'+'let'='alarmslet'
'alarmslet' anagrammed gives 'SMELL A RAT'.
(Other definitions for smell a rat that I've seen before include "Become suspicious" , "Sense something is amiss" , "Detect something suspicious" , "Suspect something's amiss" , "Suspect trickery (colloq.)" .)