They talk a lot where great rivers enter sea (3,6)

'rivers enter sea' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'they talk a lot where great' is the wordplay.
I cannot quite understand how this works, but
'talk' could be 'mouth' (synonyms) and 'mouth' is found in the answer.
'great' could be 'big' (synonyms) and 'big' is located in the answer.
A single letter 's' remains which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?