At heart (7)

I think it could be either:

Which length description are you looking for?:

central

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'at heart' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

This is the entire clue.

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

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