Metal prostheses set these off? (6)
I believe the answer is:
alarms
'set these off?' is the definition.
(alarms are set off)
'metal prostheses' is the wordplay.
'metal' becomes 'al' (chemical symbol for aluminium).
'prostheses' becomes 'arms' (prosthetic arms).
'al'+'arms'='ALARMS'
(Other definitions for alarms that I've seen before include "Danger signals" , "Frightens or alerts" , "Scares or startles" , "Disturbs" , "Clocks that wake sleepers" .)