Making a mistake pocketing a piece of jewellery (7)
I believe the answer is:
earring
'piece of jewellery' is the definition.
(I've seen this before)
'making a mistake pocketing a' is the wordplay.
'making a mistake' becomes 'erring' ('err' can be a synonym of 'make a mistake').
'pocketing' is an insertion indicator.
'erring' going around 'a' is 'EARRING'.
(Other definitions for earring that I've seen before include "Lobe pendant" , "Item of jewellery" , "Dangly ornament" , "Piece of jewellery" , "Ornament on lobe" .)