"Stop right there" (4)
(Other definitions for halt that I've seen before include "Stop, small station" , "Cease" , "game" , "Stop; minor station" , "Pull up" .)
'stop right there' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for whoa that I've seen before include "Cry for horses to stop" , "Command for horse to slow or stop" , "The King of rock and roll" , "Command to horse to stop or stand still" , "Opposite of "Giddy-up!"" .)