Talk foolishly (5)
I believe the answer is:
prate
'talk foolishly' is the definition.
(to prate is to talk nonsense)
This is all the clue.
(Other definitions for prate that I've seen before include "Taper (anag.)" , "Rabbit on" , "Talk long and foolishly" , "Talk foolishly - blab" , "Talk too much" .)