Income from flat in French street (7)
I believe the answer is:
revenue
'income' is the definition.
(revenue is a kind of income)
'flat in french street' is the wordplay.
'flat' becomes 'even' ('even' can be similar in meaning to 'flat').
'in' is an insertion indicator.
'french street' becomes 'rue' (French for street or road).
'even' put inside 'rue' is 'REVENUE'.
'from' acts as a link.
(Other definitions for revenue that I've seen before include "profit" , "Return" , "A country's income as from taxes" , "Ready" , "Takings" .)