Sorry about the dud cake (3)
I believe the answer is:
sad
'sorry' is the definition.
(I know that sorry can be written as sad)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sad that I've seen before include "A form of depression in winter, initially" , "Mournful and depressed" , "low" , "Sorrowful, unhappy" , "Melancholy, downhearted" .)