Making a mistake about a jewellery item (7)
I believe the answer is:
earring
'jewellery item' is the definition.
(I've seen this before)
'making a mistake about a' is the wordplay.
'making a mistake' becomes 'erring' ('err' can be a synonym of 'make a mistake').
'about' is an insertion indicator.
'erring' enclosing 'a' is 'EARRING'.
(Other definitions for earring that I've seen before include "Lobe pendant" , "Decoration" , "One adorning lobe" , "Aural pendant" , "Dangly ornament" .)