One of the four (or seven) sitting next to Ted (3)
I believe the answer is:
'one' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sea that I've seen before include "Large body of salt water" , "Watery expanse" , "Adriatic, for example" , "North, say" , "E.g Aral or Weddell" .)