Former currency, mostly, for French chemist (7)
I believe the answer is:
pasteur
'for french chemist' is the definition.
(I've seen this before)
'former currency mostly' is the wordplay.
'former' becomes 'past' (similar in meaning).
'currency' becomes 'euro' (currency of the Eurozone).
'mostly' means to remove the last letter (most but not all of the word).
'euro' with its final letter removed is 'eur'.
'past'+'eur'='PASTEUR'
(Other definitions for pasteur that I've seen before include "French scientist" , "He invented a method to stop milk causing disease" , "Louis --, French chemist/bacteriologist" , "Louis -, nineteenth century French chemist" , "Louis ---, Fr. bacteriologist" .)