Former regret about famous chemist (7)
I believe the answer is:
pasteur
'famous chemist' is the definition.
'Pasteur' can be an answer for 'chemist' (Pasteur is an example). I am unsure of the 'famous' bit.
'former regret about' is the wordplay.
'former' becomes 'past' (similar in meaning).
'regret' becomes 'rue' (to rue something is to regret it).
'about' shows that the letters should be reversed in order.
'rue' back-to-front is 'eur'.
'past'+'eur'='PASTEUR'
Can you help me to learn more?
(Other definitions for pasteur that I've seen before include "Louis -, nineteenth century French chemist" , "He invented a method to stop milk causing disease" , "Louis ---, Fr. bacteriologist" , "Louis --, French bacteriologist" , "Pioneer immunologist" .)