Apparition in seasonal show stealing hearts -- Marley's head (7)
I believe the answer is:
phantom
'apparition' is the definition.
(I know that apparition can be written as phantom)
'seasonal show stealing hearts marley's head' is the wordplay.
'seasonal show' becomes 'panto' (pantomime, traditional Christmas entertainment).
'stealing' is an insertion indicator.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'head' suggests taking the first letters.
The initial letter of 'marley' is 'm'.
'panto' enclosing 'h' is 'phanto'.
'phanto'+'m'='PHANTOM'
'in' acts as a link.
(Other definitions for phantom that I've seen before include "RR" , "Blush" , "Wraith" , "Ballet-dancing costume" , "Illusion (at the opera?)" .)