River, one with a drowned valley (3)
I believe the answer is:
ria
'drowned valley' is the definition.
(I've seen this before)
'river one with a' is the wordplay.
'river' becomes 'ri' (I can't explain this - if you can you should give a lot more credence to this answer).
'one with' is a charade indicator (letters next to each other).
'ri'+'a'='RIA'
Can you help me to learn more?
(Other definitions for ria that I've seen before include "Drowned river valley now an inlet of the sea" , "Little girl" , "Coastline feature, a drowned valley" , "Drowned coastal valley" , "Submerged valley" .)