Clio's sister (5)
I believe the answer is:
erato
'clio's sister' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for erato that I've seen before include "Muse of lyric poetry and mime" , "Source of poetic inspiration" , "verses she inspired" , "Clio's sister" , "Poet's inspiration" .)