Not so fast (6)
'not so fast' is the definition.
('slow' can be the opposite of 'fast' so perhaps 'slower' is reasonable for 'not so fast')
This is all the clue.
(Other definitions for slower that I've seen before include "Less able to understand" , "Not so fast!" , "taking longer" , "Being less intelligent" .)
(Other definitions for ton-up that I've seen before include "fast!" , "Travelling at one hundred mph" , "going quickly" , "At one-hundred mph" , "Going at over 100mph" .)
(Other definitions for whoa that I've seen before include "Opposite of "Giddy-up!"" , "Stop! - steady on!" , "The King of rock and roll" , "Hold your horses!" , "Said to stop a horse" .)