Herb's sort of horse (6)
I believe the answer is:
sorrel
This clue is a double definition.
'herb's' is the first definition.
(sorrel is a kind of herb)
'sort of horse' is the second definition.
(I know that sorrel is a type of Equus caballus)
(Other definitions for sorrel that I've seen before include "Edible plant of the dock family" , "It provides flavour" , "Edible plant, horse of light reddish-brown coat" , "in the stable?" , "Type of horse" .)