Ride this, trampling over everyone (9)
I believe the answer is:
'ride this' is the definition.
(as in riding roughshod over something)
I can't explain the remainder of the clue.
(Other definitions for roughshod that I've seen before include "With horseshoe nails not fully driven home" , "the domineering ride it over others" , "Like some horses" , "(Of a horse) given uneven shoes to prevent sliding" .)