Senior grabbing a cold drink — cool, sweet stuff (9)
I believe the answer is:
saccharin
'cool sweet stuff' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'senior grabbing a cold drink' is the wordplay.
'senior' becomes 'sarin' (I can't justify this - if you can you should believe this answer much more).
'grabbing' means one lot of letters goes inside another.
'a cold' becomes 'c' (eg on taps).
'drink' becomes 'cha' (cha can mean tea).
'c'+'cha'='ccha'
'sarin' enclosing 'ccha' is 'SACCHARIN'.
Can you help me to learn more?
(Other definitions for saccharin that I've seen before include "Very sweet compound" , "alternative sweetener" , "Calorie-free sweetener" , "Sweet substance used as sugar substitute" , "sweetener provided" .)