Plonk is knocked back in warehouse (7)
I believe the answer is:
deposit
'warehouse' is the definition.
(thesaurus)
'plonk is knocked back in' is the wordplay.
'plonk' becomes 'depot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'knocked back' says the letters should be written in reverse.
'in' is an insertion indicator.
'is' backwards is 'si'.
'depot' enclosing 'si' is 'DEPOSIT'.
Can you help me to learn more?
(Other definitions for deposit that I've seen before include "Down payment made to secure goods" , "store" , "Money in bank - part payment" , "Put money into a bank account" , "Set down" .)