It's a lot of money, darling (4)
I believe the answer is:
dear
'darling' is the definition.
(I know that darling can be written as dear)
'it's a lot of money' is the wordplay.
'it's a lot' becomes 'd'.
'of money' becomes 'ear' (I can't explain this - if you can you should give a lot more credence to this answer).
'd'+'ear'='DEAR'
Can you help me to learn more?
(Other definitions for dear that I've seen before include "letter opener" , "Expensive - address" , "Darling" , "At a high cost" , "Beloved - expensive" .)