River rose, forming artificial lake (9)
I believe the answer is:
reservoir
'artificial lake' is the definition.
(synonyms)
'river rose forming' is the wordplay.
'forming' indicates anagramming the letters.
'river'+'rose'='riverrose'
'riverrose' is an anagram of 'RESERVOIR'.
(Other definitions for reservoir that I've seen before include "Man-made lake or tank" , "Area flooded" , "Man-made lake to store water" , "Man-made lake of stored water" , "Large water store" .)