Essential line north - trains lack a single captain! (9)

I believe the answer is:

intrinsic

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'essential' is the definition.
(associated in meaning)

I don't understand the rest of the clue.

(Other definitions for intrinsic that I've seen before include "Natural" , "Of the essential nature of something" , "Genuine" , "Belonging as an inherent and essential part" , "native?" .)

I've seen this clue in the Sydney Morning Herald.

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