Body of water in river rose unexpectedly (9)
I believe the answer is:
reservoir
'body of water' is the definition.
(reservoir is a kind of body of water)
'river rose unexpectedly' is the wordplay.
'unexpectedly' is an anagram indicator.
'river'+'rose'='riverrose'
'riverrose' anagrammed gives 'RESERVOIR'.
'in' is the link.
(Other definitions for reservoir that I've seen before include "Lake for collecting and storing water" , "Lake used to store water for community use" , "Man-made lake or tank" , "water over a wide area?" , "Large water store" .)