Natural drug ages terribly as a sweetener (10,5)
I believe the answer is:
granulated sugar
'a sweetener' is the definition.
(I know that granulated sugar is a type of sugar)
'natural drug ages terribly' is the wordplay.
'terribly' is an anagram indicator.
'natural'+'drug'+'ages'='naturaldrugages'
'naturaldrugages' anagrammed gives 'GRANULATED SUGAR'.
'as' is the link.
(Other definitions for granulated sugar that I've seen before include "sweetener needed" , "White sweetener" .)