Blue or green area (3)
I believe the answer is:
sea
'blue' is the definition.
(thesaurus)
'green area' is the wordplay.
'green' becomes 'se' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'area' becomes 'a' (maths abbreviation).
'se'+'a'='SEA'
'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?" .)