Guide a bull (5)
I believe the answer is:
steer
This clue is a double definition.
'guide' is the first definition.
(I know that guide can be written as steer)
'a bull' is the second definition.
(I've seen this before)
(Other definitions for steer that I've seen before include "Control movement" , "Pilot a boat in desired direction" , "Direct, fix course" , "Direct; young ox" , "Guide; bullock" .)