River with poster that's never even got onto a sort of fishing (4-3)
I believe the answer is:
deep-sea
'never even got onto a sort of fishing' is the definition.
I can't tell whether this defines the answer.
'river with poster' is the wordplay.
'river' becomes 'ea' (I've seen this in other clues).
'with' says to put letters next to each other.
'poster' becomes 'deeps' (I can't explain this - if you can you should give a lot more credence to this answer).
'ea' put after 'deeps' is 'DEEP-SEA'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for deep-sea that I've seen before include "In the far ocean" , "Relating to such places as the Mariana Trench" , "Relating to remoter ocean" , "Describing a type of diving" , "This describes divers, not snorkellers!" .)