It's mainly in Asia (6,5)
I believe the answer is:
indian ocean
'asia' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'it's mainly in' is the wordplay.
I cannot quite understand how this works, but
'in' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for indian ocean that I've seen before include "Lots of water" , "Waters between Africa and Australia" , "large expanse of sea" , "Expanse of sea east of Africa" , "Region between Africa and Australia" .)