The yoke's on them (4)
![Ross](/clue/static/ross-mug.min.png)
'the yoke's on them' 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 "Steers" , "Domestic bovines" , "Domesticated bovine draft animals" , "Horned farm animals" , "Doctored bulls" .)