Described by Mach, a steady velocity (5)
I believe the answer is:
Here is my best explanation:
'velocity' is the definition.
(haste is a kind of velocity)
'described by mach a steady' is the subsidiary indication.
'described by' indicates a hidden word.
'haste' is hidden in the letters of 'mach a steady'.
(Other definitions for haste that I've seen before include "Heats (anag.)", "Expedition", "More of this can slow you up!", "The more of it, the less speed, it is said", "Overly eager speed", "Rapidity", "Rush; urgency", "Rushing (but with less speed?)", "furious activity" and "Undue speed".)