Talk at zero speed (5)
I believe the answer is:
orate
'talk' is the definition.
(to orate is to give a speech)
'at zero speed' is the wordplay.
'at' is a charade indicator (letters next to each other).
'zero' becomes 'O' (O resembles 0).
'speed' becomes 'rate' (the speed at which something happens).
'o' put next to 'rate' is 'ORATE'.
(Other definitions for orate that I've seen before include "Speak in public" , "Talk pompously" , "Speak in florid style" , "Lecture" , "Make a formal speech" .)