Seats, say, with black padding — stuff those in to fill out? (5)

I believe the answer is:

carbs

This clue could be a double definition.

'seats say' is the first definition.
(SEAT is a car manufacturer)

'stuff those in to fill out?' is the second definition.
(carbohydrates are fattening foods)

'with black padding' is the link.
This may not be correct. Some or all of it may belong to one of the definitions.

I've seen this clue in The Guardian.

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