Guided horse about to go in (7)
I believe the answer is:
steered
'guided' is the definition.
('steer' can be a synonym of 'guide')
'horse about to go in' is the wordplay.
'horse' becomes 'steed' (steed is a kind of horse).
'about to go' becomes 're' (I've seen this in other clues).
'in' is an insertion indicator.
'steed' enclosing 're' is 'STEERED'.
(Other definitions for steered that I've seen before include "Went on a course" , "given the push" , "Guided a vehicle" , "given guidance" , "Guided or directed, ship or plane" .)