Such money is for keeping mum (4)
I believe the answer is:
hush
'mum' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for hush that I've seen before include "Mum's the word" , "Make or become silent or quiet" , "Quieten" , "Call for silence" , "Sudden descent of silence" .)