Sorry the cake's flunked (3)
I believe the answer is:
sad
'sorry' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sad that I've seen before include "Sorrowful, unhappy" , "A form of depression in winter, initially" , "Mournful and depressed" , "Mournful - sorry" , "Deplorable" .)