Possibly a sugar, not something bitter (9)
I believe the answer is:
angostura
'something bitter' is the definition.
'possibly a sugar not' is the wordplay.
'possibly' is an anagram indicator.
'a'+'sugar'+'not'='asugarnot'
'asugarnot' with letters rearranged gives 'ANGOSTURA'.
(Other definitions for angostura that I've seen before include "bitter source" , "An aromatic bark, made bitter tonic" , "Bitters for 13" , "bitter substance" , "It makes 8 pink" .)