Seats, say, with black padding — stuff those in to fill out? (5)
I believe the answer is:
Here is my best explanation:
'stuff those in to fill out?' is the definition.
(carbohydrates are fattening foods)
'seats say with black' is the subsidiary indication.
'seats say' becomes 'cars' (I can't explain this - if you can you should believe this answer much more).
'with' indicates putting letters inside.
'black' becomes 'b' (as in pencil classifications).
'cars' going around 'b' is 'carbs'.
'padding' is the link.
I am not very happy about this link. It may be part of another bit of the clue.