Quiet boy getting hold of vehicle poster (7)
I believe the answer is:
placard
'poster' is the definition.
(I know that poster can be written as placard)
'quiet boy getting hold of vehicle' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'boy' becomes 'lad' (informal term for boy).
'getting hold of' means one lot of letters goes inside another.
'vehicle' becomes 'car' (car is a kind of vehicle).
'lad' going around 'car' is 'lacard'.
'p'+'lacard'='PLACARD'
(Other definitions for placard that I've seen before include "One taken on march" , "Make card, pal, into kind of ad" , "Sign for public display" , "what may be carried in a demo" , "Notice carried for public display" .)