Measure and add (3)
I believe the answer is:
tot
I believe this clue is a double definition.
'measure' is the first definition.
(I have seen 'Measure of spirits' mean 'tot' so perhaps 'measure' could also mean 'tot')
'add' is the second definition.
(I know that add can be written as tot)
'and' acts as a link.
(Other definitions for tot that I've seen before include "Add; small child" , "Sum for the little one" , "A wee dram" , "Drop" , "Small drink - young child" .)