Briefed on the first day (6)

I believe the answer is:

primed

'briefed' is the definition.
(I've seen this before)

'the first day' is the wordplay.
'the first' becomes 'prime' (I've seen this before).
'day' becomes 'd' (abbreviation for day).
'prime'+'d'='PRIMED'

'on' is the link.

(Other definitions for primed that I've seen before include "Filled in" , "charged" , "Made ready for action" , "Briefed - ready" , "Prepared something for use or action" .)

I've seen this clue in the Evening Standard.

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