Answer: alerting

AnswerCrossword Clue
ALERTINGPutting on guard
ALERTINGSiren's function
ALERTINGEmulating Paul Revere
ALERTINGPaul Revere's midnight duty
ALERTINGRed flag's purpose
ALERTINGTipping off
ALERTINGSounding the alarm
alertingALERT, to warn (to notify in advance)
alertingWarn (someone) of a danger, threat, or problem, typically with the intention of having it avoided or dealt with