Pompous colonel is an old gasbag (5)
I believe the answer is:
blimp
This clue might be a double definition.
'pompous colonel' is the first definition.
'blimp' can be an answer for 'colonel' (cartoon character Colonel Blimp). I am not sure about the 'pompous' bit.
'an old gasbag' is the second definition.
'blimp' can be an answer for 'gasbag' (thesaurus). I'm unsure of the 'an old' bit.
'is' is the link.
Can you help me to learn more?
(Other definitions for blimp that I've seen before include "returns one pound to provost" , "aircraft" , "Small barrage balloon" , "Small nonrigid airship" , "Airship - barrage balloon" .)