Number of Muses (4)
I believe the answer is:
nine
'number of muses' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nine that I've seen before include "Days Lady Jane 5 ac reigned" , "One's married partner" , "One over the eight" , "SECOND PART OF 14" , "One more than fourteen across" .)