Sign mostly put on a road (7)
I believe the answer is:
placard
'sign' is the definition.
(I know that placard is a type of sign)
'mostly put on a road' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'road' could be 'rd' (abbreviation) and 'rd' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for placard that I've seen before include "Printed notice" , "Make card, pal, into kind of ad" , "what may be carried in a demo" , "Bill" , "Large portable notice" .)