Wistfulness (6)
I believe the answer is:
sorrow
'wistfulness' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for sorrow that I've seen before include "'Sadness, grief (6)'" , "Sadness, feeling of regret" , "Sadness; one magpie for it" , "'One magpie for ..., two for joy (6)'" , "Shakespeare: ''Parting is such sweet . . . . . .'' (6)" .)