Calliope's sister (5)
I believe the answer is:
erato
'calliope's sister' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for erato that I've seen before include "Greek muse of lyric and love poetry" , "One of the Muses" , "Clio's sister" , "goddess" , "Poet's inspiration" .)