Special ceremony is featured in writer’s activity (of medium interest?) (12)

'activity of medium interest?' is the definition.
(spiritualism might include an interest in mediums)

'special ceremony is featured in writer's' is the wordplay.
'special' becomes 'sp'.
'ceremony' becomes 'ritual' (I've seen this before).
'featured in' is an insertion indicator.
'writer's' becomes 'im' ('writer's' could mean 'the crossword setter is').
'ritual'+'is'='ritualis'
'ritualis' inserted inside 'im' is 'iritualism'.
'sp'+'iritualism'='SPIRITUALISM'