Accountant's on top of hush money (4)
I believe the answer is:
cash
'money' is the definition.
(I've seen this before)
'accountant's on top of hush' is the wordplay.
'accountant's' becomes 'cas' (abbreviation for chartered accountant).
'on top of' suggests taking the first letters (I've seen 'on top' mean this).
The initial letter of 'hush' is 'h'.
'cas'+'h'='CASH'
(Other definitions for cash that I've seen before include "Money in the form of notes and coins" , "Hard currency" , "Ready money" , "bread" , ""Readies"" .)