Setters heartlessly upset by bullocks (6)
I believe the answer is:
steers
'bullocks' is the definition.
('steer' can be a synonym of 'bullock')
'setters heartlessly upset by' is the wordplay.
'heartlessly' says to hollow out the word (remove centre letters).
'upset by' is an anagram indicator.
'setters' with its middle removed is 'seters'.
'seters' is an anagram of 'STEERS'.
(Other definitions for steers that I've seen before include "Guides a ship or car" , "Guides or directs, ship or plane" , "Determines the direction" , "Controls the direction" , "Guides direction of, ship or plane" .)