Not just a little bit of money (4,3)
I believe the answer is:
lump sum
'not just a little' is the definition.
I can't tell whether this defines the answer.
'bit of money' is the wordplay.
'bit of' becomes 'lump' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'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 "Large single payment" , "chunk of bread?" , "Large payment, often in cash" , "Whole payment" , "cash payout" .)