Guided a horse holding half the rein (7)
I believe the answer is:
steered
'guided' is the definition.
('steer' can be a synonym of 'guide')
'a horse holding half the rein' is the wordplay.
'a horse' becomes 'steed' (steed is a kind of horse).
'holding' means one lot of letters goes inside another.
'half the' suggests halving the letters.
'rein' halved is 're'.
'steed' enclosing 're' is 'STEERED'.
(Other definitions for steered that I've seen before include "given the push" , "Went on a course" , "taken the helm" , "Guided or directed, ship or plane" , "given guidance" .)