Regret accepting equal income (7)
I believe the answer is:
revenue
'income' is the definition.
(revenue is a kind of income)
'regret accepting equal' is the wordplay.
'regret' becomes 'rue' (synonyms).
'accepting' is an insertion indicator.
'equal' becomes 'even' (similar in meaning).
'rue' enclosing 'even' is 'REVENUE'.
(Other definitions for revenue that I've seen before include "Return" , "A country's income as from taxes" , "profit" , "Government's income" , "Money coming in" .)