This has sea to south and east (5)
I believe the answer is:
maine
'this' is the definition.
The definition suggests a singular noun which matches the answer.
'sea to south and east' is the wordplay.
I cannot quite see how this works, but
'sea' could be 'main' (synonyms) and 'main' is present in the answer.
'east' could be 'e' (abbreviation) and 'e' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'has' acts as a link.
Can you help me to learn more?
(Other definitions for maine that I've seen before include "American state, capital Augusta" , "North-east American state" , "state of USA" , "New England state" , "Part of America" .)