The French consumed by Callas fever (7)
I believe the answer is:
malaria
'fever' is the definition.
(I've seen this before)
'the french consumed by callas' is the wordplay.
'the french' becomes 'la' ('the' in French).
'consumed by' is an insertion indicator (inserted letters get consumed).
'callas' becomes 'maria' (Maria Callas).
'la' inserted inside 'maria' is 'MALARIA'.
(Other definitions for malaria that I've seen before include "Infectious fever" , "Protozoal disease" , "Disease causing fever transmitted by mosquitos" , "Disease from bites by infected mosquitos" , "Mosquito-borne disease" .)