The most common and most likely answer to this crossword clue is the 5 letter word ASTOR
I've seen this crossword clue in The Daily Telegraph
A
S
T
O
R
More clues leading to the result ASTOR
- Topping minister, pioneering politician
- Notable first female priest's powerless
- Former lady in Parliament? Bright star, but no leader
- Female MP once in a never-ending rage
- Former female MP with a tale that never ends
- House-sitter, first female one since coming to prominence
- Reactionary corrupts a pioneering MP
- Powerless minister? Pioneering MP!
Buddy explains!
The straight part
We know that "Pioneering MP" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pioneering mp = astor.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.