Top side making down-payment (7)
I believe the answer is:
deposit
'down-payment' is the definition.
(I've seen this before)
'top side making' is the wordplay.
'top side' becomes 'topside' (synonyms).
'making' indicates anagramming the letters.
'topside' with letters rearranged gives 'DEPOSIT'.
(Other definitions for deposit that I've seen before include "Sum of money paid into a bank account" , "store" , "Down payment made to secure goods" , "secure an item" , "Lodge money in bank account" .)