Crossword Genius

The nicest possible cover for a rain tank (7)

Ross

I believe the answer is:

cistern

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'tank' is the definition.
(cistern is a kind of tank)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for cistern that I've seen before include "Water storage tank" , "Tank - sac of body fluid" , "Vat" , "WC water tank" , "Tank, reservoir" .)

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