Zeus turned her to stone (5)
I believe the answer is:
'zeus turned her to stone' 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 "Daughter of Tantalus whose children were all slain" , "proud mum" , "Petrified" , "Greek woman cast in stone" , "woman of sorrows" .)