Nonsense! An alarm is set off by the key (8)
I believe the answer is:
malarkey
'nonsense' is the definition.
(I've seen this before)
'an alarm is set off by the key' is the wordplay.
'is set off' indicates anagramming the letters.
'by' says to put letters next to each other.
'alarm' with letters rearranged gives 'malar'.
'malar'+'key'='MALARKEY'
(Other definitions for malarkey that I've seen before include "Absurd talk" , "Nonsensical stuff" , "Idle words" , "Cock" , "Slangy humbug or nonsense - sounds like Irish name" .)