Assume extremists for years have backed council (5)

I believe the answer is:

synod

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'council' is the definition.
(I know that Synod is a council)

I don't understand how the remainder of the clue works.

(Other definitions for synod that I've seen before include "Ecclesiastical gathering" , "Ecclesiastic council" , "Ecclesiastical assembly" , "Forum for discussing" , "Church council" .)

I've seen this clue in The Times.

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