Poster of a card to the port authority (7)
I believe the answer is:
placard
'poster' is the definition.
(I know that poster can be written as placard)
'a card to the port authority' is the wordplay.
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'the port authority' becomes 'pla' (Port of London Authority).
'card' put after 'pla' is 'PLACARD'.
'of' is the link.
(Other definitions for placard that I've seen before include "what may be carried in a demo" , "Sign for public display" , "Bill" , "Printed notice" , "Slogan or message of protest on board" .)