Blue or green area (3)
I believe the answer is:
sea
This clue could be a double definition.
'blue' is the first definition.
(thesaurus)
'green area' is the second definition.
The answer and definition can be both natural objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'or' is the link.
Can you help me to learn more?
(Other definitions for sea that I've seen before include "Body of saltwater" , "The main" , "Lesser ocean" , "Ocean area" , "that's easily reached from Dover?" .)