At heart, most important (7)

I believe the answer is:

central

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

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

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

(Other definitions for central that I've seen before include "Near the middle" , "Extremely important" , "like this answer" , "Most important, principal" , "Key" .)

I've seen this clue in the Evening Standard.

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