the very beginning (6)
'the very beginning' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Another definition for day one that I've seen is " On which an operation begins".)
'the very beginning' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for origin that I've seen before include "Source, beginning point" , "First thing" , "base" , "Where axes cut" , "Where X meets Y" .)