Let down seeing scenery set in river (7)
I believe the answer is:
deflate
'let down' is the definition.
(thesaurus)
'scenery set in river' is the wordplay.
'scenery' becomes 'flat' (flat is a kind of scenery).
'set in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'flat' going inside 'dee' is 'DEFLATE'.
'seeing' acts as a link.
(Other definitions for deflate that I've seen before include "Release gas from" , "Puncture, collapse like balloon" , "Take air" , "Release air from" , "Cause to shrink or collapse" .)