Warning triangle is broken (8)
I believe the answer is:
Here is my best explanation:
'warning' is the definition.
(alerting is a kind of warning)
'triangle is broken' is the subsidiary indication.
'is broken' indicates anagramming the letters.
'triangle' with letters rearranged gives 'alerting'.
(Other definitions for alerting that I've seen before include "Warning" and "Notifying".)