That lady's selfless successor (3)
I believe the answer is:
her
'that lady's' is the definition.
('her' refers to a woman)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for her that I've seen before include "She, objectively" , "The woman's" , "The female's" , "That lady's" , "Pronoun used of ship" .)