Chatterbox (6)
I believe the answer is:
gasbag
'chatterbox' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gasbag that I've seen before include "Loudmouth - or balloon?" , "A compulsive talker with balloon?" , "One going on" , "He likes to talk" , "I say too much" .)