Beginning completed over a year (3,3)

I believe the answer is:

day one

'beginning' is the definition.
(the first day of something)

'completed over a year' is the wordplay.
'completed' becomes 'done' (something that's been completed has been done).
'over' indicates putting letters inside.
'year' becomes 'y'.
'a'+'y'='ay'
'done' placed around 'ay' is 'DAY ONE'.

(Another definition for day one that I've seen is " On which an operation begins".)

I've seen this clue in The Guardian.

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