Writer initially lost following old motorway sign (4)
I believe the answer is:
omen
'sign' is the definition.
(omen is a kind of sign)
'writer initially lost following old motorway' is the wordplay.
'writer' becomes 'pen' (something which writes).
'initially lost' means to remove the first letter.
'following' says to put letters next to each other.
'old motorway' becomes 'om' (this might be a standard abbreviation I've not previously seen).
'pen' with its initial letter taken off is 'en'.
'en' after 'om' is 'OMEN'.
Can you help me to learn more?
(Other definitions for omen that I've seen before include "Portent, prophetic indicator" , "Sign of things to come" , "Augur" , "Auspice" , "Portent of things to come" .)