Cocktail: mix mine at the bottom, then do the same to yours? (7,4)
I believe the answer is:
whiskey sour
'cocktail' is the definition.
(whiskey sour is a kind of cocktail)
'mix mine at the bottom then do the same to yours?' is the wordplay.
I cannot really see how this works, but
'then' could be 'so' (both can mean 'therefore' or 'in that case') and 'so' is found in the answer.
This explanation may well be incorrect...
(Other definitions for whiskey sour that I've seen before include "Cocktail" , "drink" .)