Crossword Genius

Key for a trumpet

Ross

I believe the answer is:

valve

(Other definitions for valve that I've seen before include "Device regulating flow" , "Structure with a flap to insure one-way flow of fluid" , "Device for controlling the flow of a fluid in a pipe" , "Part of brass instrument" , "Fluid regulator" .)

I've seen this clue in The New York Times.
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