French bacteriologist (7)
I believe the answer is:
'french bacteriologist' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pasteur that I've seen before include "He invented a method to stop milk causing disease" , "French chemist, 1822-95" , "Louis --, French bacteriologist" , "French chemist and biologist, d. 1895" , "Louis -, nineteenth century French chemist" .)