Observance at heart of spiritualism (6)
I believe the answer is:
ritual
'observance' is the definition.
(I have seen 'Customary observance' mean 'ritual' so perhaps 'observance' could also mean 'ritual')
'heart of spiritualism' is the wordplay.
'heart of' says the answer is hidden in the clue.
'RITUAL' is hidden within 'spiritualism'.
'at' acts as a link.
(Other definitions for ritual that I've seen before include "Ceremony or service" , "Ceremony or protocol" , "Actions performed as in a religious ceremony" , "Ceremony following a prescribed order" , "Regular stereotyped action" .)