Bag with tea in, and sweetener (9)
I believe the answer is:
saccharin
'sweetener' is the definition.
(saccharin is a kind of sweetener)
'bag with tea in' is the wordplay.
'bag' becomes 'sac' (I've seen this before).
'with' means one lot of letters go next to another.
'tea' becomes 'char' (I've seen this before).
'sac'+'char'+'in'='SACCHARIN'
'and' is the link.
(Other definitions for saccharin that I've seen before include "Very sweet compound" , "Sweet substance used as sugar substitute" , "Artificial sweetener" , "sweetener provided" , "Synthetic sweetener" .)