Sort of killer in a thriller? (6)

I believe the answer is:

serial

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'sort of killer' is the definition.

I can't explain the remainder of the clue.

(Other definitions for serial that I've seen before include "Episodic story from Israel" , "Story published in parts" , "Program that's often on" , "Story made up of consecutive parts" , "In a sequence" .)

I've seen this clue in the Evening Standard.

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