River or sea perhaps, not a lake (9)
I believe the answer is:
reservoir
'lake' is the definition.
(reservoir is a kind of lake)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for reservoir that I've seen before include "Lake used to store water for community use" , "Large water store" , "Lake created to store water" , "Man-made lake of stored water" , "Man-made lake to store water" .)