Have suspicions, lets alarm off (5,1,3)
I believe the answer is:
smell a rat
'have suspicions' is the definition.
'lets alarm off' is the wordplay.
'off' is an anagram indicator.
'lets'+'alarm'='letsalarm'
'letsalarm' is an anagram of 'SMELL A RAT'.
(Other definitions for smell a rat that I've seen before include "Suspect trickery (colloq.)" , "Detect something suspicious" , "Become suspicious" , "Sense something suspicious" , "Suspect something wrong" .)