Security is returning to enter warehouse (7)
I believe the answer is:
deposit
'security' is the definition.
(deposit is a kind of security)
'is returning to enter warehouse' is the wordplay.
'returning' says the letters should be written backwards.
'to enter' means one lot of letters goes inside another.
'warehouse' becomes 'depot' (I've seen this before).
'is' backwards is 'si'.
'si' placed within 'depot' is 'DEPOSIT'.
(Other definitions for deposit that I've seen before include "(Money or mineral) put down" , "First instalment" , "Place" , "Put money into a bank account" , "Lay down" .)