Case rotten under dish in salt water (6,3)
I believe the answer is:
baltic sea
'salt water' is the definition.
(I've seen this before)
'case rotten under dish' is the wordplay.
'rotten' indicates an anagram.
'under' says to put letters next to each other.
'dish' becomes 'balti' (I've seen this before**).
'case' is an anagram of 'csea'.
'csea' put after 'balti' is 'BALTIC SEA'.
'in' is the link.
(Other definitions for baltic sea that I've seen before include "large amount of water" , "Salt water" , "watery expanse" , "Water between Poland and Scandinavia" .)