Big cash payment - not even money (4,3)
I believe the answer is:
lump sum
'big cash payment' is the definition.
'lump sum' can be an answer for 'payment' (I have seen 'Single payment' mean 'lump sum' so perhaps 'payment' could also mean 'lump sum'). I'm not sure about the 'big cash' bit.
'not even money' is the wordplay.
'not even' becomes 'lump' (I can't explain this - if you can you should believe this answer much more).
'money' becomes 'sum' (I've seen this in other clues).
'lump'+'sum'='LUMP SUM'
Can you help me to learn more?
(Other definitions for lump sum that I've seen before include "Whole payment" , "Large single payment" , "Single payment as opposed to a number of smaller ones" , "Large payment, often in cash" , "Single payment as opposed to many instalments" .)