Swimmer in river crazy about Dutch port (9)
I believe the answer is:
rotterdam
'dutch port' is the definition.
(I've seen this before)
'swimmer in river crazy about' is the wordplay.
'swimmer' becomes 'otter' (animal which swims).
'in' is an insertion indicator.
'river' becomes 'r'.
'crazy' becomes 'mad' (synonyms).
'about' says the letters should be written backwards.
'mad' back-to-front is 'dam'.
'r'+'dam'='rdam'
'otter' placed within 'rdam' is 'ROTTERDAM'.
(Other definitions for rotterdam that I've seen before include "Major North Sea port - tarred tom (anag)" , "Netherlands port" , "city abroad" , "One of the world's largest ports" , "Dutch city and port" .)