Regret going west to follow former French chemist (7)
I believe the answer is:
pasteur
'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' is a reversal indicator (letters heading to the west or left).
'to follow' means one lot of letters go next to another (some letters go after or follow others).
'former' becomes 'past' (similar in meaning).
'rue' reversed gives 'eur'.
'eur' put 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" .)