Sad Mr James, funny fellow, being stood up then engaged (12)
I believe the answer is:
disappointed
'sad' is the definition.
'mr james funny fellow being stood up then engaged' is the wordplay.
'mr james funny fellow' becomes 'sid' (Sid James).
'being stood up' says the letters should be written in reverse (in a down clue, letters go up).
'then' is a charade indicator (letters next to each other).
'engaged' becomes 'appointed'.
'sid' backwards is 'dis'.
'dis'+'appointed'='DISAPPOINTED'
(Other definitions for disappointed that I've seen before include "Sad and let-down" , "Feeling sad" , "Let down" , "Let-down, frustrated" , "Unhappy" .)