Sister of Regan and Cordelia (7)
I believe the answer is:
goneril
'sister of regan and cordelia' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for goneril that I've seen before include "part 1 of 25 [KING LEAR]" , "Daughter of seven dn" , "One of Lear's daughter" , "Female character in King Lear" , "Woman" .)