Slumping having made poor comeback and decidedly disheartened (6)
I believe the answer is:
droopy
'disheartened' is the definition.
(thesaurus)
'slumping having made poor' is the wordplay.
'slumping' becomes 'dy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'having' is an insertion indicator.
'made' indicates anagramming the letters.
'poor' anagrammed gives 'roop'.
'dy' placed around 'roop' is 'DROOPY'.
'comeback and decidedly' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for droopy that I've seen before include "Lacking strength" , "Tending to 27 [SAG]" , "Cartoon dog - sagging" , "Hanging limply - animated dog created by Tex Avery" , "Inclined to sag" .)