Spit in drink (6)
I believe the answer is:
double
I believe this clue is a double definition.
'spit' is the first definition.
(thesaurus)
'drink' is the second definition.
(I have seen 'Large drink' mean 'double' so perhaps 'drink' could also mean 'double')
'in' acts as a link.
(Other definitions for double that I've seen before include "Person who says one thing and does another" , "- - -u, (w)" , "Dartboard outer score" , "Offence of leaving one's car where it blocks another" , "Spitting image" .)