Communion the very thing, trapped by disease supposedly (7)
I believe the answer is:
liturgy
'communion' is the definition.
'the very thing trapped by disease supposedly' is the wordplay.
'the very thing' becomes 'it'.
'trapped by' is an insertion indicator.
'disease supposedly' becomes 'lurgy'.
'it' going inside 'lurgy' is 'LITURGY'.
(Other definitions for liturgy that I've seen before include "church office" , "Form of religious service" , "ritual" , "Prescribed forms for church services" , "Form of worship" .)