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' (both can mean pace).
'o'+'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" .)