A step taken fast (5)
I believe the answer is:
apace
'fast' is the definition.
(apace can mean quickly)
'a step' is the wordplay.
'step' becomes 'pace' (pace is a kind of step).
'a'+'pace'='APACE'
'taken' acts as a link.
(Other definitions for apace that I've seen before include "At a rapid rate" , "at high speed" , "fast" , "Quick" , "What's quickly" .)