Pudding that’s no good (4)
I believe the answer is:
duff
This clue is a double definition.
'pudding' is the first definition.
(duff is a kind of pudding)
'no good' is the second definition.
(I've seen this before)
'that's' acts as a link.
(Other definitions for duff that I've seen before include "Homer's so-called favourite tipple" , "False; steamed pudding" , "Pudding -- useless -- Homer Simpson's kind of beer" , "Heavy pudding - no good" , "Heavy boiled or steamed pudding" .)