Quickly taking one step (5)
I believe the answer is:
apace
'quickly' is the definition.
(I know that quickly can be written as apace)
'one step' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'step' becomes 'pace' (synonyms).
'a'+'pace'='APACE'
'taking' is the link.
(Other definitions for apace that I've seen before include "What's quickly" , "at high speed" , "Quick" , "At a rapid rate" , "fast" .)