Person bringing action in quarrel with port authority (9)
I believe the answer is:
plaintiff
'person bringing action' is the definition.
'in quarrel with port authority' is the wordplay.
'quarrel' becomes 'tiff' (tiff is a kind of quarrel**).
'with' means one lot of letters go next to another.
'port authority' becomes 'pla' (Port of London Authority).
'in'+'tiff'='intiff'
'intiff' after 'pla' is 'PLAINTIFF'.
(Other definitions for plaintiff that I've seen before include "Person initiating lawsuit" , "Bringer of lawsuit" , "Person bringing a civil action in a court of law" , "The litigant making the accusation" , "Person bringing a legal action" .)