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' means one lot of letters go next to another.
'zero' becomes 'O' (O resembles 0).
'speed' becomes 'rate' (both can mean pace).
'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" .)