Horse-stopping shout (4)
I believe the answer is:
whoa
'horse-stopping shout' 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 "Instruction to a horse to stop" , "Danger" , "Opposite of "Giddy-up!"" , "Command to horse to stop or stand still" , "Order (horse) to stop" .)