Topping minister, pioneering politician (5)
I believe the answer is:
astor
'pioneering politician' is the definition.
(Nancy Astor, first female MP to attend parliament)
'topping minister' is the wordplay.
'topping' means to remove the first letter (top can mean to remove the top from something).
'minister' becomes 'pastor' (synonyms).
'pastor' with its initial letter taken away is 'ASTOR'.
(Other definitions for astor that I've seen before include "First woman to sit in the House of Commons, d. 1964" , "Female politician" , "female parliamentarian" , "First female MP" , "See 2" .)