A tree, of course, and a horse (5,8)
I believe the answer is:
sweet chestnut
'a horse' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'a tree of course' is the wordplay.
I cannot really understand how this works, but
'tree' could be 'chestnut' (chestnut tree is a kind of tree) and 'chestnut' is located in the answer.
'course' could be 'sweet' (sweet is a kind of course) and 'sweet' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'and' is the link.
Can you help me to learn more?
(Other definitions for sweet chestnut that I've seen before include "Edible tree seed" , "Tree whose fruit gets roasted" , "Tree with edible nuts" .)