Regret about flat income (7)
I believe the answer is:
revenue
'income' is the definition.
(revenue is a kind of income)
'regret about flat' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'about' indicates putting letters inside.
'flat' becomes 'even' ('even' can be similar in meaning to 'flat').
'rue' placed around 'even' is 'REVENUE'.
(Other definitions for revenue that I've seen before include "profit" , "Gross business income" , "Takings" , "Earnings" , "stream, perhaps" .)