Covered wagon pullers (4)
I believe the answer is:
oxen
'covered wagon pullers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oxen that I've seen before include "Bovine animals, in yoke" , "Domestic bovines" , "Horned farm animals" , "Draught animals" , "Heavy cattle" .)