Down payment is back in store (7)
I believe the answer is:
deposit
'down payment' is the definition.
(synonyms)
'is back in store' is the wordplay.
'back' says the letters should be written backwards.
'in' is an insertion indicator.
'store' becomes 'depot' (depot can mean a store or warehouse).
'is' back-to-front is 'si'.
'si' placed into 'depot' is 'DEPOSIT'.
(Other definitions for deposit that I've seen before include "First instalment" , "Lodge money in bank account" , "(Money) laid down" , "Posited as down payment" , "Place" .)