Crossword Genius

At the heart of the tube system (7)

Ross

I believe the answer is:

central

I'm a little stuck... Click here to teach me more about this clue!

This might be a double definition.

'at' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'heart of the tube system' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'the' acts as a link.

Can you help me to learn more?

(Other definitions for central that I've seen before include "Focal" , "Key" , "Middle; crucial" , "Every answer here has one such letter" , "like this answer" .)

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