Store outside Santander's affirmative bank (7)
I believe the answer is:
deposit
'bank' is the definition.
(synonyms)
'store outside santander's affirmative' is the wordplay.
'store' becomes 'depot' (depot can mean a store or warehouse).
'outside' means one lot of letters goes inside another.
'santander's affirmative' becomes 'si' (I can't explain this - if you can you should believe this answer much more).
'depot' enclosing 'si' is 'DEPOSIT'.
Can you help me to learn more?
(Other definitions for deposit that I've seen before include "is one back on the bottle?" , "Sum of money paid into a bank account" , "fur, possibly" , "Part payment in advance to secure goods" , "secure an item" .)