Errol's wild horse (6)
I believe the answer is:
sorrel
'horse' is the definition.
(sorrel is a kind of horse)
'errol's wild' is the wordplay.
'wild' indicates anagramming the letters.
'errols' with letters rearranged gives 'SORREL'.
(Other definitions for sorrel that I've seen before include "Edible plant, horse of light reddish-brown coat" , "Perennial herb" , "Light chestnut" , "Edible plant of the dock family" , "Light-chestnut horse" .)