Lady who's breaking rule (4,4)
I believe the answer is:
hold sway
'rule' is the definition.
(I've seen this in another clue)
'lady who's breaking' is the wordplay.
'breaking' indicates anagramming the letters.
'lady'+'whos'='ladywhos'
'ladywhos' is an anagram of 'HOLD SWAY'.
(Other definitions for hold sway that I've seen before include "Have the upper hand, rule - sadly who?" , "Have power and influence over - how sadly?" .)