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It's made to react to pressure (9)

Ross

I believe the answer is:

barometer

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'pressure' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

I don't understand the rest of the clue.

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(Other definitions for barometer that I've seen before include "Weatherglass" , "It checks state of sky" , "Air-pressure measurer" , "Instrument to measure atmospheric pressure" , "Instrument that measures atmospheric pressure" .)

I've seen this clue in The Irish Independent.
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