Notice young man secures vehicle after parking (7)
I believe the answer is:
placard
'notice' is the definition.
(synonyms)
'young man secures vehicle after parking' is the wordplay.
'young man' becomes 'lad' (lad can mean a boy or young man).
'secures' means one lot of letters goes inside another.
'vehicle' becomes 'car' (car is a kind of vehicle).
'after' says to put letters next to each other.
'parking' becomes 'p' (abbreviation on road signs).
'lad' placed around 'car' is 'lacard'.
'lacard' after 'p' is 'PLACARD'.
(Other definitions for placard that I've seen before include "Notice carried for public display" , "Sign for public display" , "Poster" , "Bill" , "what may be carried in a demo" .)