Inflationary cause of the balloon going up? (3)

I believe the answer is:

gas

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'inflationary cause' is the definition.
The definition and answer can be both to do with communicating as well as being verbs in their base form.
Maybe there's a link between them I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for gas that I've seen before include "Eg, oxygen" , "Chatter (colloq.)" , "Something that is neither solid not liquid" , "Fuel; idle talk" , "Americans' petrol" .)

I've seen this clue in the Evening Standard.

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