Leading lady curtailed test in protest (9)
I believe the answer is:
matriarch
'leading lady' is the definition.
(a matriarch is a woman who leads)
'curtailed test in protest' is the wordplay.
'curtailed' means to remove the last letter.
'test' becomes 'trial' (synonyms).
'in' is an insertion indicator.
'protest' becomes 'march' (marching is a kind of protesting).
'trial' with its final letter taken off is 'tria'.
'tria' put into 'march' is 'MATRIARCH'.
(Other definitions for matriarch that I've seen before include "venerable old lady" , "Dominating female" , "Dominant female" , "Very old or venerable woman" , "Powerful older woman" .)