Regret going west to follow former French chemist (7)
I believe the answer is:
'french chemist' is the definition.
(I've seen this before)
'regret going west to follow former' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'going west' says the letters should be written in reverse (letters heading to the west or left).
'to follow' is a charade indicator (letters next to each other).
'former' becomes 'past' (similar in meaning).
'rue' back-to-front is 'eur'.
'eur' after 'past' is 'PASTEUR'.
(Other definitions for pasteur that I've seen before include "He invented a method to stop milk causing disease" , "Louis -, nineteenth century French chemist" , "Louis --, French chemist and microbiologist" , "He developed the partial sterilization of milk" , "French chemist and biologist, d. 1895" .)