Monsieur Pasteur (5)
I believe the answer is:
louis
'monsieur pasteur' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for louis that I've seen before include "-- Armstrong, trumpeter" , "French monarch's name" , "-- Armstrong, singer; Joe --, boxer" , "Name of many Fr. kings" , "Joe ---, the Brown Bomber" .)