Gibber (5)
I believe the answer is:
prate
'gibber' is the definition.
(I know that gibber can be written as prate)
This is all the clue.
(Other definitions for prate that I've seen before include "Taper (anag.)" , "Talk long and foolishly" , "Rabbit on" , "Tattle, chatter" , "Talk too much" .)