Crossword Genius

Open or close it to control the flow of a liquid or gas (5)

Ross

I believe the answer is:

valve

(Other definitions for valve that I've seen before include "Structure with a flap to ensure one-way flow" , "Flow controller" , "Device regulating flow" , "Flow regulator" , "Structure with a flap to insure one-way flow of fluid" .)

I've seen this clue in The Guardian.
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