The nicest possible cover for a rain tank (7)

I believe the answer is:

cistern

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'tank' is the definition.
(I know that cistern is a type of reservoir)

I can't explain the remainder of the clue.

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

I've seen this clue in the Evening Standard.

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