Otherwise it's part of a chapel service (4)
I believe the answer is:
else
'otherwise' is the definition.
(synonyms)
'part of a chapel service' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ELSE' is hidden within 'chapel service'.
'it's' acts as a link.
(Other definitions for else that I've seen before include "Adverb meaning if not, otherwise" , "Besides, otherwise" , "Besides; instead" , "In addition; different" , "As an alternative" .)