Walked upon (4)
'walked upon' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trod that I've seen before include "Took steps" , "Walked heavily, put foot down" , "Pressed (grapes) by foot" , "Crushed (grapes) by foot" , "Trudged, stepped" .)
(Other definitions for trodden that I've seen before include "Stepped on" , "Crushed underfoot" , "Oppressed" , "Crushed under the feet" , "Heavily walked upon" .)