Thus get US attorney a drink (4)
I believe the answer is:
soda
'a drink' is the definition.
(I know that soda is a type of soft drink)
'thus get us attorney' is the wordplay.
'thus' becomes 'so' (synonyms).
'get' means one lot of letters go next to another.
'us attorney' becomes 'DA' (abbreviation for District Attorney).
'so'+'da'='SODA'
(Other definitions for soda that I've seen before include "Mixer drink" , "Whisky mixer" , "fizz" , "Alkaline substance used in cleaning, drinks etc" , "Type of bread, water or fountain" .)