Below the speed of sound (8)
I believe the answer is:
subsonic
'below the speed of sound' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for subsonic that I've seen before include "Such low rates for Jumbos" , "Under 768 mph" , "At a speed less than that of sound" , "Less speedy than sound" , "Of speed lower than that of sound" .)