It's imperative to one of those in The Office (9)

I believe the answer is:

incumbent

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'it's imperative to one of those' is the definition.
I can't judge whether this defines the answer.

'in the office' is the wordplay.
'the office' becomes 'cumbent' (I am not sure about this - if you are sure you should believe this answer much more).
'in'+'cumbent'='INCUMBENT'

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(Other definitions for incumbent that I've seen before include "official?" , "Mandatory" , "Currently holding an office" , "What's obligatory" , "Functionary" .)

I've seen this clue in the Irish Times.

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