Door, open -- so ___? Profit! (6)
I believe the answer is:
income
'profit' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for income that I've seen before include "What do you make" , "Money received as wages, interest etc" , "Money from investments or for work" , "See 12" , "Money received on which tax is payable" .)