River rose turning into lake (9)
I believe the answer is:
reservoir
'lake' is the definition.
(reservoir is a kind of lake)
'river rose turning' is the wordplay.
'turning' indicates anagramming the letters.
'river'+'rose'='riverrose'
'riverrose' anagrammed gives 'RESERVOIR'.
'into' is the link.
(Other definitions for reservoir that I've seen before include "Man-made lake or tank" , "Lake created to store water" , "Store for water etc" , "Area for storage, of water say" , "Source of water supply" .)