One’s piercingly going off the mark for an adornment (7)
I believe the answer is:
earring
'an adornment' is the definition.
(earring is a kind of adornment)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for earring that I've seen before include "Decoration" , "Ornament on lobe" , "Lobe pendant" , "connection between girl and pearl" , "Piece of jewellery" .)