Wild horse overly accelerating initially - stop! (4)
I believe the answer is:
whoa
'stop' is the definition.
(I've seen this before)
'wild horse overly accelerating initially' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'wild horse overly accelerating' is 'WHOA'.
(Other definitions for whoa that I've seen before include "Call to stop" , "Opposite of "Giddy-up!"" , "Command for horse to slow or stop" , "Cry for horses to stop" , "Said to stop a horse" .)