Believe one side of the account (6)
I believe the answer is:
credit
I believe this is a double definition.
'believe' is the first definition.
(I know that believe is a more specific form of credit)
'one side of the account' is the second definition.
(I've seen this before)
(Other definitions for credit that I've seen before include "Money available for a client to borrow" , "Time to pay, or due recognition" , "In this, you have money in the bank" , "Honour or reliability - believe" , "entry in cashbook" .)