Alerting light (5)
I believe the answer is:
flare
'alerting light' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flare that I've seen before include "Burst into sudden activity or anger" , "Warning light" , "(Burn with) sudden blaze" , "Spread" , "Distress signal; burst out angrily" .)