Chief supporter to the bitter end (6)

I believe the answer is:

master

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'chief' is the definition.
(I know that chief can be written as master)

I can't explain the remainder of the clue.

(Other definitions for master that I've seen before include "Lord - controller - teacher" , "Main; overcome; original recording" , "An old one is a great European painter" , "with 1a [NEW] Jacky may have in nursery" , "With 1a [OLD], one who painted" .)

I've seen this clue in the Evening Standard.

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