Puzzling person or thing (6)
I believe the answer is:
enigma
This clue is a double definition.
'puzzling person' is the first definition.
(I've seen this in another clue)
'thing' is the second definition.
(I have seen 'Mysterious thing ' mean 'enigma' so perhaps 'thing' could also mean 'enigma')
(Other definitions for enigma that I've seen before include "Mysterious person/thing" , "WW2 code" , "Title of Elgar's set of variations" , "Mysterious person" , "cryptic puzzle!" .)