Turn north-east after old motorway sign (4)
I believe the answer is:
omen
'sign' is the definition.
(I know that omen is a type of sign)
'turn north-east after old motorway' is the wordplay.
'turn' says the letters should be written in reverse.
'north east' becomes 'ne'.
'after' is a charade indicator (letters next to each other).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'ne' back-to-front is 'en'.
'o'+'m'='om'
'en' after 'om' is 'OMEN'.
(Other definitions for omen that I've seen before include "Sign of things to come" , "premonition" , "Sign of something to come" , "Warning token" , "Straw in the wind" .)