Trample (5)
'trample' is the definition.
(I know that trample can be written as tread)
This is the entire 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".)