Trample (5)
'trample' is the definition.
(synonyms)
This is all the clue.
(Other definitions for tread that I've seen before include "Sole of a shoe" , "pace" , "Tyre moulding" , "between the car and the road" , "Surface of car tyre" .)
(Another definition for stepon that I've seen is " Treat harshly".)