Mad retort about port (9)
I believe the answer is:
rotterdam
'port' is the definition.
(I've seen this before)
'mad retort about' is the wordplay.
'about' indicates anagramming the letters.
'mad'+'retort'='madretort'
'madretort' is an anagram of 'ROTTERDAM'.
(Other definitions for rotterdam that I've seen before include "Netherlands port" , "European port" , "Major North Sea port - tarred tom (anag)" , "foreign city" , "European city" .)