Mathematician from Swiss city getting French agreement to cover lines (9)
I believe the answer is:
bernoulli
'mathematician' is the definition.
(I've seen this before)
'swiss city getting french agreement to cover lines' is the wordplay.
'swiss city' becomes 'bern' (Bern is the capital of Switzerland).
'getting' means one lot of letters go next to another.
'french agreement' becomes 'oui' (I've seen this in another clue).
'to cover' is an insertion indicator.
'lines' becomes 'll' (an abbreviation for line written twice).
'oui' enclosing 'll' is 'oulli'.
'bern'+'oulli'='BERNOULLI'
'from' is the link.
(Other definitions for bernoulli that I've seen before include "Physicist" , "Mathematician" .)