Crossword Genius

Principal line in London (7)

Ross

I believe the answer is:

central

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'principal' is the definition.
(I've seen this before)

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

Can you help me to learn more?

(Other definitions for central that I've seen before include "Middle; crucial" , "Focal" , "Near the middle" , "Most important, principal" , "Extremely important" .)

I've seen this clue in The Mirror.
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