It indicates it's time to be on the alert (5)
I believe the answer is:
watch
'it indicates it's time to' is the definition.
The definition suggests a singular noun which matches the answer.
'on the alert' is the wordplay.
I cannot really see how this works, but
'on' could be 'at' (I've seen this in other clues) and 'at' is found in the answer.
This explanation may well be incorrect...
'be' acts as a link.
Can you help me to learn more?
(Other definitions for watch that I've seen before include "Keep an eye on - timepiece" , "period of duty" , "Timepiece - look out" , "Observe - timepiece" , "Vigil" .)