Move fast, initially getting in step (4)
I believe the answer is:
rung
'step' is the definition.
(thesaurus)
'move fast initially getting' is the wordplay.
'move fast' becomes 'run' (I've seen this before).
'initially' says to take the initial letters.
The initial letter of 'getting' is 'g'.
'run'+'g'='RUNG'
'in' is the link.
(Other definitions for rung that I've seen before include "Telephoned" , "Horizontal bar in ladder" , "spoke" , "support for foot" , "Phoned" .)