Mythical queen of Thebes (5)
I believe the answer is:
'mythical queen of thebes' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for niobe that I've seen before include "figure of myth" , "Tantalus's daughter" , "she wept for her children" , "Weeping woman" , "proud mum" .)