Ghostly double (12)
I believe the answer is:
doppelganger
'ghostly double' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for doppelganger that I've seen before include "Spirit double" , "A person's spitting image" , "Ghostly double that haunts its living counterpart" , "Living person's double" , "Ghostly double of a living person - green Lapp god (anag)" .)