"Ars Poetica" poet (6)
I believe the answer is:
'ars poetica poet' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for horace that I've seen before include "Odist" , "Is it a chore to read this old Roman poet?" , "Roman poet; -- Walpole" , "Latin ode writer" , "- Walpole, eighteenth century English author" .)