The sediment is back in the warehouse (7)
I believe the answer is:
deposit
'the sediment' is the definition.
(synonyms)
'is back in the warehouse' is the wordplay.
'back' says the letters should be written backwards.
'in the' means one lot of letters goes inside another.
'warehouse' becomes 'depot' (I've seen this before).
'is' backwards is 'si'.
'si' inserted within 'depot' is 'DEPOSIT'.
(Other definitions for deposit that I've seen before include "First instalment" , "Sum of money paid into a bank account" , "Precipitate" , "Put money into a bank account" , "Posited as down payment" .)