Sister of Calliope (5)
I believe the answer is:
erato
'sister of calliope' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for erato that I've seen before include "One of the Muses" , "Poet's inspiration" , "goddess" , "Poetic Muse ; eighteen (anag.)" , "Muse of 11 and 23" .)