Dwelling in river is let down (7)
I believe the answer is:
deflate
'is let down' is the definition.
(I've seen this before)
'dwelling in river' is the wordplay.
'dwelling' becomes 'flat' (I can't justify this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'flat' placed inside 'dee' is 'DEFLATE'.
Can you help me to learn more?
(Other definitions for deflate that I've seen before include "Puncture, collapse like balloon" , "Cause to shrink or collapse" , "Let the wind out of, braggart or balloon" , "Let air out of, like balloon" , "Take air" .)