Operated by air pressure (9)
I believe the answer is:
pneumatic
'operated by air pressure' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pneumatic that I've seen before include "Operated by pressurised gas" , "form of drill?" , "Using compressed air" , "Inflated with air" , "Driven by compressed air" .)