Sorry about that cake (3)

I believe the answer is:

sad

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

'sorry' is the definition.
(I know that sorry can be written as sad)

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

(Other definitions for sad that I've seen before include "Gloomy" , "Regrettable" , "Melancholy, downhearted" , "Miserable" , "Blue" .)

I've seen this clue in the Evening Standard.

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