Chatter quietly at speed (5)
I believe the answer is:
prate
'chatter' is the definition.
(I know that chatter can be written as prate)
'quietly at speed' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'at' says to put letters next to each other.
'speed' becomes 'rate' (the speed at which something happens).
'p'+'rate'='PRATE'
(Other definitions for prate that I've seen before include "Talk too much" , "Rabbit on" , "Tattle, chatter" , "Talk long and foolishly" , "Taper (anag.)" .)