Stick with old city microbiologist (7)
I believe the answer is:
pasteur
'microbiologist' is the definition.
(I know that Pasteur has the occupation microbiologist)
'stick with old city' is the wordplay.
'stick' becomes 'paste' (as in sticking or pasting something down).
'with' means one lot of letters go next to another.
'old city' becomes 'ur' (ancient Sumerian city state).
'paste'+'ur'='PASTEUR'
(Other definitions for pasteur that I've seen before include "Louis --, French chemist/bacteriologist" , "French chemist and biologist, d. 1895" , "Louis -, nineteenth century French chemist" , "French scientist" , "French chemist and biologist (1822-1895)" .)