Velocity (5)
'velocity' is the definition.
(I know that velocity can be written as speed)
This is all the clue.
(Other definitions for speed that I've seen before include "Rapidity of movement" , "A devilish quick driver?" , "Swiftness" , "Rapidity - amphetamine" , "Rate of movement" .)
(Other definitions for scrum that I've seen before include "brawl" , "Disorderly struggle" , "Disorderly mob" , "A pushy rugby formation" , "A rugby play" .)