Balloon for a chatterbox (6)
I believe the answer is:
gasbag
This clue is a double definition.
'balloon' is the first definition.
(thesaurus)
'for a chatterbox' is the second definition.
(I've seen this before)
(Other definitions for gasbag that I've seen before include "An informal blatherer" , "One going on" , "One talks incessantly about trivial things" , "bore" , "I talk too much" .)