Reservoir (3)
'reservoir' is the definition.
(source of water)
This is the entire clue.
(Other definitions for tank that I've seen before include "Large storage chamber" , "defeat" , "Offensive vehicle" , "Armoured vehicle - container for liquid or gas" , "Storage chamber; drink heavily (colloq.)" .)
'reservoir' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for dam that I've seen before include "eg Aswan" , "Weir" , "Horse's mother" , "River blockage" , "River restraint" .)