All the same as an all-seater? (15)

I believe the answer is:

notwithstanding

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'all the same' is the definition.
('notwithstanding' can be a synonym of 'all the same')

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

Can you help me to learn more?

(Other definitions for notwithstanding that I've seen before include "Regardless of" , "Despite" , "Nevertheless" , "without being still" , "Even so" .)

I've seen this clue in The Telegraph.

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