Described by Mach, a steady velocity (5)
I believe the answer is:
haste
'velocity' is the definition.
(haste is a kind of velocity)
'described by mach a steady' is the wordplay.
'described by' indicates the answer is hidden within the clue.
'HASTE' is hidden in the letters of 'mach a steady'.
(Other definitions for haste that I've seen before include "Speediness or rush" , "Rushing (but with less speed?)" , "velocity" , "The more of it, the less speed sometimes" , "go at speed" .)