Regret about following former chemist (7)
I believe the answer is:
pasteur
'chemist' is the definition.
(I know that Pasteur is a chemist)
'regret about following former' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'about' says the letters should be written backwards.
'following' means one lot of letters go next to another.
'former' becomes 'past' (similar in meaning).
'rue' back-to-front is 'eur'.
'eur' put after 'past' is 'PASTEUR'.
(Other definitions for pasteur that I've seen before include "French chemist, 1822-95" , "Louis --, French chemist/bacteriologist" , "Louis --, French chemist and microbiologist" , "He developed the partial sterilization of milk" , "Pioneer immunologist" .)