Storage tank (9)
I believe the answer is:
reservoir
'storage tank' is the definition.
(I know that reservoir is a type of storage tank)
This is the entire clue.
(Other definitions for reservoir that I've seen before include "Lake for collecting and storing water" , "River rose (anag.)" , "Lake used to store water for community use" , "Store for water etc" , "Area for storage, of water say" .)