Speed is ace (5)
I believe the answer is:
whizz
I believe this is a double definition.
'speed' is the first definition.
(whizzing is a kind of speeding)
'ace' is the second definition.
(I know that ace can be written as whizz)
'is' is the link.
(Other definitions for whizz that I've seen before include "Move rapidly (colloq.)" , "Skilled person (colloq.)" , "Person with great skill" , "Hum - buzz - expert" , "Loud humming - extremely skilful person" .)