One who says a lot about a balloon (6)
I believe the answer is:
gasbag
'balloon' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for gasbag that I've seen before include "One who talks too much, or a balloon" , "Talkative person or filled balloon" , "One talks at length on boring topics" , "a container of helium" , "An informal blatherer" .)