Why is the good and bad bittersweet? (5,10)
I believe the answer is:
woody nightshade
'bittersweet?' is the definition.
('woody nightshade' can be a synonym of 'bittersweet')
'why is the good and bad' is the wordplay.
'bad' indicates anagramming the letters.
'why'+'is'+'the'+'good'+'and'='whyisthegoodand'
'whyisthegoodand' anagrammed gives 'WOODY NIGHTSHADE'.
(Other definitions for woody nightshade that I've seen before include "plant" , "bittersweet?" .)