Thus get attorney a drink (4)
I believe the answer is:
soda
'a drink' is the definition.
(soda is a kind of drink)
'thus get attorney' is the wordplay.
'thus' becomes 'so' (synonyms).
'get' means one lot of letters go next to another.
'attorney' becomes 'DA' (district attorney).
'so'+'da'='SODA'
(Other definitions for soda that I've seen before include "In the US, a sweet fizzy drink" , "pressurised liquid" , "Alkaline substance used in cleaning, drinks etc" , "Splash served with some spirits" , "Accompaniment to whisky" .)