Bullock in herd (5)
I believe the answer is:
steer
This clue is a double definition.
'bullock' is the first definition.
('steer' can be a synonym of 'bullock')
'herd' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for steer that I've seen before include "Young bull" , "Handle" , "Guide (a ship, car, etc)" , "Guide, control" , "Cattle - guide" .)