Aluminium provides protection to warning devices (6)
I believe the answer is:
alarms
'to warning devices' is the definition.
('alarm' can be a synonym of 'warning device')
'aluminium provides' is the wordplay.
'aluminium' becomes 'al' (Al is the chemical symbol for aluminium).
'provides' becomes 'arms' (arming is a kind of providing).
'al'+'arms'='ALARMS'
'protection' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for alarms that I've seen before include "Frightens like warning devices" , "Danger signals" , "is scary" , "Puts on guard or frightens" , "Frightens, makes afraid" .)