Speed sounds high! (3)
I believe the answer is:
hie
'speed' is the definition.
(I know that speed can be written as hie)
'sounds high' is the wordplay.
'sounds' shows a homophone (sound like).
'high' is a homophone of 'HIE'.
(Other definitions for hie that I've seen before include "Go quickly (archaic)" , "Hasten" , "Make one's way quickly" , "make haste" , "Hurry" .)