... I'm annoyed about symbol of power missing by mill (7)

I believe the answer is:

grinder

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'mill' is the definition.
('grinder' can be a synonym of 'mill')

I don't understand the rest of the clue.

(Other definitions for grinder that I've seen before include "Molar tooth" , "Mill; molar" , "Eg, pepper mill" , "Operator of 12 across" , "Machine tool to remove jagged edges" .)

I've seen this clue in The Telegraph.

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