Hidden among sheep, a certain sort of horse (5)
I believe the answer is:
pacer
'sort of horse' is the definition.
'hidden among sheep a certain' is the subsidiary indication.
'hidden' indicates a hidden word.
'pacer' is hidden in the letters of 'among sheep a certain'.
(Other definitions for pacer that I've seen before include "Horse; one setting speed" , "walker?" , "One who sets the running speed; horse" , "setting the rate" , "One setting race speed" .)