Yankee's after a mixer for drink (6)
I believe the answer is:
Here is my best explanation:
'drink' is the definition.
(whisky is a kind of drink)
'yankee's after a mixer' is the subsidiary indication.
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'after' means one lot of letters go next to another.
'a mixer' becomes 'whisk' (whisk is a kind of mixer).
'y' put after 'whisk' is 'whisky'.
'for' is the link.
(Other definitions for whisky that I've seen before include "Spirit distilled from malted grain", "Alcoholic drink", "Scottish spirit", "reportedly available for Thin Lizzy gig?", "Grain spirit", "Malted grain spirit", "Drink", "Scotch", "Grain alcohol" and "W in radio alphabet".)