Hymn Horace composed about Ligarius originally (7)
I believe the answer is:
chorale
'hymn' is the definition.
(I know that chorale is a type of hymn)
'horace composed about ligarius originally' is the wordplay.
'composed' is an anagram indicator.
'about' is an insertion indicator.
'originally' says to take the initial letters.
The initial letter of 'ligarius' is 'l'.
'horace' anagrammed gives 'chorae'.
'chorae' going around 'l' is 'CHORALE'.
(Other definitions for chorale that I've seen before include "Simple hymn tune" , "Psalm or hymn tune" , "Harmonised hymn tune" , "Composition" , "Lutheran hymn-tune" .)