Sister of Erato (4)
I believe the answer is:
clio
'sister of erato' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for clio that I've seen before include "inspiration for historians" , "One of the Muses" , "Muse of history - car" , "one of nine sisters" , "In Greek and Roman myth, the Muse of history" .)