Fusses about popular mixer (4)
I believe the answer is:
soda
'popular mixer' is the definition.
(I've seen this in another clue)
'fusses about' is the wordplay.
'fusses' becomes 'ados' ('ado' can be a synonym of 'fuss').
'about' is a reversal indicator.
'ados' reversed gives 'SODA'.
(Other definitions for soda that I've seen before include "Splash served with some spirits" , "Fizzy drink - chemical compound" , "Alkaline substance used in cleaning, drinks etc" , "Variety of pop" , "Mixer drink" .)