Scoundrel joining mother in Dutch port (9)
I believe the answer is:
rotterdam
'dutch port' is the definition.
(I've seen this before)
'scoundrel joining mother' is the wordplay.
'scoundrel' becomes 'rotter' (both can mean a nasty person).
'joining' is a charade indicator (letters next to each other).
'mother' becomes 'dam' (female parent of an animal).
'rotter'+'dam'='ROTTERDAM'
'in' acts as a link.
(Other definitions for rotterdam that I've seen before include "One of the world's largest ports" , "Dutch city and port" , "European city" , "Major North Sea port - tarred tom (anag)" , "Major world seaport at the mouth of the Meuse" .)