Christian sacrament for Italian succumbing to undiagnosed illness (7)
I believe the answer is:
liturgy
'christian sacrament' is the definition.
'italian succumbing to undiagnosed illness' is the wordplay.
'italian' becomes 'it'.
'succumbing to' is an insertion indicator.
'undiagnosed illness' becomes 'lurgy' (**).
'it' going into 'lurgy' is 'LITURGY'.
'for' acts as a link.
(Other definitions for liturgy that I've seen before include "Prescribed forms for church services" , "Form of religious service" , "Standard form of church service" , "Church rite" , "Form of worship" .)