Upright post as part of a frame — do arm job (anag) (8)

I believe the answer is:

doorjamb

(Other definitions for doorjamb that I've seen before include "Entranceway post" , "Entrance sidepost" , "Side post at entrance" , "Side of an entrance's framework" , "Vertical post by entrance" .)

I've seen this clue in The Guardian.

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