Doctor sips at aperitif (6)
I believe the answer is:
pastis
'aperitif' is the definition.
'doctor sips at' is the wordplay.
'doctor' indicates anagramming the letters.
'sips'+'at'='sipsat'
'sipsat' with letters rearranged gives 'PASTIS'.
(Other definitions for pastis that I've seen before include "Rectangular" , "Alcoholic drink flavoured with aniseed" , "Aniseed-flavour aperitif" , "Aniseed aperitif" , "Aniseed-flavoured aperitif" .)