Reservoir (4)

I think it could be either:

Which length description are you looking for?:

tank

'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.)" .)

I've seen this clue in The Mirror and The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.