Main line in London (7)

I believe the answer is:

central

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'main' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

(Other definitions for central that I've seen before include "Every answer here has one such letter" , "Near the middle" , "Middle; crucial" , "Most important, principal" , "Key" .)

I've seen this clue in The Daily Express.

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