Place one's found in river to stand in (8)
I believe the answer is:
deputise
'stand in' is the definition.
(I've seen this before)
'place one's found in river' is the wordplay.
'place' becomes 'put' ('put' can be a synonym of 'place').
'one's' becomes 'is' (roman numeral).
'found in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'put'+'is'='putis'
'putis' put within 'dee' is 'DEPUTISE'.
'to' acts as a link.
(Other definitions for deputise that I've seen before include "Stand in for" , "Appoint as representative" , "Appoint as a substitute" , "Act as delegate for a person" , "Appoint as stand-in" .)