The most common and most likely answer to this crossword clue is the 7 letter word UNSEATS
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Buddy explains!
The straight part
We know that "What new MP does to old one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what new mp does to old one = unseats.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.
What does UNSEATS mean?
UNSEATS verb- To throw from one's seat; to deprive of a seat.
- To deprive of the right to sit in a legislative body, as for fraud in election, or simply by defeating them in an election.