Warning of imminent danger (3,5)
I believe the answer is:
red alert
'warning of imminent danger' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for red alert that I've seen before include "Urgent warning" , "Scarlet warning of imminent danger?" , "State of readiness for imminent crisis" , "Severe 1 dn" , "State of readiness for imminent threat" .)