Decorator’s accessory: staff retaining piece to copy (7,4)
I believe the answer is:
Here is my best explanation:
'decorator's accessory' is the definition.
'staff retaining piece to copy' is the subsidiary indication.
'staff' becomes 'mast'.
'retaining' is an insertion indicator.
'piece' becomes 'king' (king is a kind of piece**).
'to copy' becomes 'ape' ('ape' can be a synonym of 'copy').
'mast' enclosing 'king' is 'maskingt'.
(Another definition for masking tape that I've seen is "I keep coat off".)