Notice vehicle protected by youth under pressure (7)
I believe the answer is:
placard
'notice' is the definition.
(I know that notice can be written as placard)
'vehicle protected by youth under pressure' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'protected by' means one lot of letters goes inside another (inserted letters are held or protected).
'youth' becomes 'lad' (lad means a young man).
'under' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'pressure' becomes 'p' (scientific abbreviation).
'car' put inside 'lad' is 'lacard'.
'lacard' after 'p' is 'PLACARD'.
(Other definitions for placard that I've seen before include "Poster" , "Slogan or message of protest on board" , "Notice carried for public display" , "Printed notice" , "One taken on march" .)