One pitched in river for easing of tension (7)
I believe the answer is:
detente
'easing of tension' is the definition.
(I've seen this before)
'one pitched in river' is the wordplay.
'one pitched' becomes 'tent' (an item pitched on a campsite).
'in' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'tent' put into 'dee' is 'DETENTE'.
'for' acts as a link.
(Other definitions for detente that I've seen before include "peace during hostilities" , "Easing of international tension" , "Easing of strained relations" , "Easing of international hostility" , "Easing of hostilities" .)