French mathematician’s friend upset doctor (7)
I believe the answer is:
laplace
'french mathematician's' is the definition.
(Pierre-Simon Laplace, 1749-1827)
'friend upset doctor' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'upset' shows that the letters should be reversed in order.
'doctor' becomes 'lace' (doctor can mean to lace food or drink).
'pal' reversed gives 'lap'.
'lap'+'lace'='LAPLACE'
(Other definitions for laplace that I've seen before include "Pierre-Simon -, Fr. mathematician, astronomer" , "French scientist" , "French astronomer and mathematician" .)