Rider's command (4)
I believe the answer is:
whoa
'rider's command' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for whoa that I've seen before include "Order (horse) to stop" , "Opposite of "Giddy-up!"" , "Call to stop" , "horsy expression?" , "The King of rock and roll" .)