Cause of inflation? (3)

I believe the answer is:

air

'cause of inflation?' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for air that I've seen before include "Refrain" , "Atmosphere surrounding the earth" , "Means of travelling" , "in which birds fly around" , "Tune - mood - atmosphere" .)

I've seen this clue in the Penny Press, The Sun and the LA Times.

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