Through which the river goes into the sea? (4,8)

'through which the river goes into' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'sea?' is the wordplay.
I cannot quite see how this works, but
'sea?' could be 'main' (synonyms) and 'main' is found within the answer.
The remaining letters 'entrance' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'the' acts as a link.

Can you help me to learn more?