Chatterbox (6)
(Other definitions for talker that I've seen before include "Speaker" , "Rat" , "conversationalist?" .)
'chatterbox' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gasbag that I've seen before include "Excessive chatterer" , "I talk too much" , "bore" , "One talks at length on boring topics" , "Prattler" .)