"Step on it!” old-style (3)
I believe the answer is:
hie
'step on it old-style' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hie that I've seen before include "Go quickly (archaic)" , "Hasten" , "Hurry" , "make haste" , "Be quick (lit.)" .)