It’s a profitable difference (4-2)
I believe the answer is:
mark-up
'difference' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I can't see?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for mark-up that I've seen before include "Margin of profit" , "The amount added to the cost to determine the selling price" , "Ghost" , "Difference between cost and asking price" , "The amount added to the cost to arrive at the selling price" .)