The French get commonplace disease (7)
I believe the answer is:
leprosy
'disease' is the definition.
(leprosy is a kind of health problem)
'the french get commonplace' is the wordplay.
'the french' becomes 'le'.
'get' is a charade indicator (letters next to each other).
'commonplace' becomes 'prosy'.
'le'+'prosy'='LEPROSY'
(Other definitions for leprosy that I've seen before include "Reply so (anag.)" , "Disfiguring disease" , "Infectious disease of the skin" , "Disease affecting skin and nerves" , "Hansen's disease" .)