Rough seas do for Black Sea port (6)
I believe the answer is:
odessa
'black sea port' is the definition.
(I've seen this before)
'rough seas do' is the wordplay.
'rough' indicates an anagram.
'seas'+'do'='seasdo'
'seasdo' is an anagram of 'ODESSA'.
'for' is the link.
(Other definitions for odessa that I've seen before include "Ukrainian Black Sea port" , "Ukrainian naval base" , "Do seas come into Ukrainian city?" , "Port city on the Black Sea" , "Ukrainian location" .)