Endlessly rubbishing what Horace wrote (5)
I believe the answer is:
latin
'what horace wrote' is the definition.
'endlessly rubbishing' is the wordplay.
'endlessly' means to look at the middle letters (the ends of the words are removed).
'rubbishing' becomes 'slating'.
The middle of 'slating' is 'LATIN'.
(Other definitions for latin that I've seen before include "and 15 Down: Student area of Paris" , "Old Roman language" , "Shakespeare's was small, according to Jonson" , "Language of old Romans" , "difficult subject" .)