Poster to position shortly over a road (7)
I believe the answer is:
placard
'poster' is the definition.
('placard' can be a synonym of 'poster')
'position shortly over a road' is the wordplay.
'position shortly' becomes 'calp' (I can't justify this - if you can you should give a lot more credence to this answer).
'over' says the letters should be written in reverse.
'road' becomes 'rd' (abbreviation).
'calp' in reverse letter order is 'plac'.
'plac'+'a'+'rd'='PLACARD'
'to' is the link.
(Other definitions for placard that I've seen before include "what may be carried in a demo" , "Printed notice" , "Make card, pal, into kind of ad" , "One taken on march" , "Sign posted in a public place as an advertisement" .)