Sextet in function backing old Roman poet (4)
I believe the answer is:
ovid
'old roman poet' is the definition.
(I've seen this in another clue)
'sextet in function backing' is the wordplay.
'sextet' becomes 'VI' (synonyms).
'in' means one lot of letters goes inside another.
'function' becomes 'do' (I've seen this in other clues).
'backing' is a reversal indicator.
'do' backwards is 'od'.
'vi' put within 'od' is 'OVID'.
(Other definitions for ovid that I've seen before include "Vi, do read the Latin poet" , "Roman known for his love poetry" , "Writer" , "Void for old Roman poet" , "Latin poet (Ars amatoria, Metamorphoses)" .)