Sorry to be a bit sadistic (3)
I believe the answer is:
sad
'sorry' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sad that I've seen before include "Winter blues, initially" , "A form of depression in winter, initially" , "Miserable" , "Doleful" , "upset" .)