Chemist over regret, on reflection (7)
I believe the answer is:
pasteur
'chemist' is the definition.
(I know that Pasteur is a chemist)
'over regret on reflection' is the wordplay.
'over' becomes 'past' (both can mean finished).
'regret' becomes 'rue' (to rue something is to regret it).
'on reflection' shows that the letters should be reversed in order.
'rue' backwards is 'eur'.
'past'+'eur'='PASTEUR'
(Other definitions for pasteur that I've seen before include "Famous bacteriologist" , "Louis --, French chemist/bacteriologist" , "Louis -, nineteenth century French chemist" , "French chemist and biologist (1822-1895)" , "Louis --, French chemist and microbiologist" .)