To cancel Mass, say, at end of day is a bit of a nerve (8)
I believe the answer is:
dendrite
'a bit of a nerve' is the definition.
(part of a nerve cell)
'to cancel mass say at end of day' is the wordplay.
'to cancel' becomes 'end'.
'mass say' becomes 'rite' (Mass is a type of church rite).
'at end of' says to put letters next to each other (some letters go at the end of others).
'day' becomes 'd' (abbreviation for day).
'end'+'rite'='endrite'
'endrite' after 'd' is 'DENDRITE'.
'is' is the link.
(Other definitions for dendrite that I've seen before include "Crystalline tree-like structure" , "Dried net of the nerve fibre" .)