Saint troubled when joining in religious feast, showing dislike of ceremony (10)
I believe the answer is:
puritanism
'dislike of ceremony' is the definition.
'saint troubled when joining in religious feast' is the wordplay.
'troubled' indicates anagramming the letters.
'when joining in' is an insertion indicator.
'religious feast' becomes 'purim' (Jewish festival).
'saint' is an anagram of 'tanis'.
'tanis' put into 'purim' is 'PURITANISM'.
'showing' is the link.
(Other definitions for puritanism that I've seen before include "Protestant movement arising in the late C 16" , "creed?" .)