Charming thing replacing British leader in PM once with leader in multimillionaire (4)
I believe the answer is:
mojo
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for mojo that I've seen before include "American charm" , "Magical influence" , "Magic charm or power in African-American culture, as heard in R&B music" , "Magical power" .)