That old gas-bag of a colonel (5)
I believe the answer is:
blimp
'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" .)