Denounce river container (8)
I believe the answer is:
execrate
'denounce' is the definition.
(I've seen this before)
'river container' is the wordplay.
'river' becomes 'exe' (River Exe).
'container' becomes 'crate' (crate is a type of container).
'exe'+'crate'='EXECRATE'
(Other definitions for execrate that I've seen before include "Harshly criticise" , "Denounce or detract" , "Curse" , "Damn" , "'Abhor, find repugnant (8)'" .)