Devoted follower became a hater in the end (8)

I believe the answer is:

adherent

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'devoted follower' is the definition.
'adherent' can be an answer for 'follower' (adherent is a kind of follower). I'm unsure of the 'devoted' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for adherent that I've seen before include "sticker" , "Sticking; partisan" , "Person supporting" , "One supports" , "Supporter; sticking" .)

I've seen this clue in The Irish Times.

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