Begin observing magpies every night - it's a sign (4)
I believe the answer is:
omen
'a sign' is the definition.
(omen is a kind of sign)
'begin observing magpies every night' is the wordplay.
'begin' says to take the initial letters.
The first letters of 'observing magpies every night' is 'OMEN'.
'it's' acts as a link.
(Other definitions for omen that I've seen before include "Gloomy sign" , "Foreboding" , "Warning token" , "Event regarded as a prophetic sign of good or evil" , "Portent, prophetic sign" .)