Down payment is returned in store (7)
I believe the answer is:
deposit
'down payment' is the definition.
('deposit' can be a synonym of 'down payment')
'is returned in store' is the wordplay.
'returned' says the letters should be written backwards.
'in' is an insertion indicator.
'store' becomes 'depot' ('depot' can be a synonym of 'store').
'is' in reverse letter order is 'si'.
'si' going into 'depot' is 'DEPOSIT'.
(Other definitions for deposit that I've seen before include "Precipitate" , "Returnable sum" , "Money given as security" , "(Money or mineral) put down" , "Put into, bank account say" .)