It's made to react to pressure (9)
I believe the answer is:
barometer
'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.
Can you help me to learn more?
(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" .)