Crossword Genius

That old gas-bag of a colonel (5)

Ross

I believe the answer is:

blimp

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'colonel' is the definition.
(cartoon character Colonel Blimp)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for blimp that I've seen before include "Small nonrigid airship" , "Elderly pompous reactionary person" , "Airship - barrage balloon" , "returns one pound to provost" , "aircraft" .)

I've seen this clue in the Evening Standard.
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