Drink for lookalike (6)
I believe the answer is:
double
This clue is a double definition.
'drink' is the first definition.
(double shot of alcohol)
'look-alike' is the second definition.
(I've seen this in another clue)
'for' acts as a link.
(Other definitions for double that I've seen before include "Two times" , "Offence of leaving one's car where it blocks another" , "Second charge for the same offence" , "Twice - bent over - run" , "Ringer - large drink" .)