Regret accepting level income (7)
I believe the answer is:
revenue
'income' is the definition.
(I know that income can be written as Revenue)
'regret accepting level' is the wordplay.
'regret' becomes 'rue' (synonyms).
'accepting' means one lot of letters goes inside another.
'level' becomes 'even' (similar in meaning).
'rue' placed around 'even' is 'REVENUE'.
(Other definitions for revenue that I've seen before include "stream, perhaps" , "Earnings" , "Income from tax" , "A country's income as from taxes" , "Government's income" .)