What's the betting? Not a lump sum (4,5)
I believe the answer is:
even money
'sum' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I don't see?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for even money that I've seen before include "Kind of bet where you stand to win or lose the sum wagered" , "likely to happen!" , "Betting odds offering chance to win the amount wagered" , "A good chance" , "Odds offering an equal chance of winning or losing" .)