Security is regressive in storehouse (7)
I believe the answer is:
deposit
'security' is the definition.
(deposit is a kind of security)
'is regressive in storehouse' is the wordplay.
'regressive' says the letters should be written in reverse.
'in' indicates putting letters inside.
'storehouse' becomes 'depot' ('depot' can be a synonym of 'storehouse').
'is' back-to-front is 'si'.
'si' put within 'depot' is 'DEPOSIT'.
(Other definitions for deposit that I've seen before include "Put down - leave behind" , "Part payment in advance to secure goods" , "Put down or leave something" , "Sum of money paid into a bank account" , "(Money) laid down" .)