A source of unwellness in northern ocean? (6)
I believe the answer is:
nausea
This clue could be a double definition.
'a source of unwellness' is the first definition.
(nausea means feeling sick)
'northern ocean?' is the second definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'in' is the link.
(Other definitions for nausea that I've seen before include "Queasy feeling" , "Sick feeling" , "Malady" , "Squeamishness" , "Ill-feeling" .)