At heart, blatherer condemns sweeping description of marine area (3,4)
I believe the answer is:
the deep
'description of marine area' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'at heart blatherer condemns sweeping' is the wordplay.
'at heart' indicates the central letters.
'condemns sweeping' becomes 'deep' (I can't explain this - if you can you should give a lot more credence to this answer).
The central letters of 'blatherer' are 'the'.
'the'+'deep'='THE DEEP'
Can you help me to learn more?
(Another definition for the deep that I've seen is " LITERATURE".)