In a river or a rushing stream (7)
I believe the answer is:
torrent
'a rushing stream' is the definition.
(I've seen this before)
'in a river or' is the wordplay.
'in' indicates putting letters inside.
'a river' becomes 'trent' (River Trent).
'trent' enclosing 'or' is 'TORRENT'.
(Other definitions for torrent that I've seen before include "Heavy flow of water" , "Swift or violent stream" , "Turbulent, swift-flowing stream" , "Overwhelming flow" , "Turbulent flow" .)