Crossword Genius

Be suspicious when alarm's let off (5,1,3)

Ross

I believe the answer is:

smell a rat

'be suspicious' is the definition.
(I've seen this before)

'alarm's let off' is the wordplay.
'off' indicates an anagram.
'alarms'+'let'='alarmslet'
'alarmslet' with letters rearranged gives 'SMELL A RAT'.

'when' acts as a link.

(Other definitions for smell a rat that I've seen before include "Suspect trickery (colloq.)" , "Sense something suspicious" , "Suspect something wrong" , "Detect something suspicious" , "Become suspicious" .)

I've seen this clue in The Telegraph.
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