British islands forbidding reservoir (7)

I believe the answer is:

cistern

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

'british islands forbidding' is the wordplay.
'british islands' becomes 'CI' (Channel Islands).
'forbidding' becomes 'stern' (synonyms).
'ci'+'stern'='CISTERN'

(Other definitions for cistern that I've seen before include "WC water tank" , "Toilet tank [ital]" , "Water storage tank" , "Tank, reservoir" , "Vat" .)

I've seen this clue in The Guardian.

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