Spirit that’s a double (12)
I believe the answer is:
doppelganger
'double' is the definition.
(I've seen this in another clue)
'spirit that's a' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for doppelganger that I've seen before include "Living person's double" , "Ghostly double that haunts its living counterpart" , "A person's spitting image" , "Spirit double" , "Ghostly double of a living person - green Lapp god (anag)" .)