Aniseed-flavoured aperitif (6)
'aniseed-flavoured aperitif' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pastis that I've seen before include "Alcoholic drink flavoured with aniseed" , "Aniseed aperitif" , "Rectangular" , "Aniseed-flavour aperitif" , "Aniseed-flavoured aperitif" .)
(Other definitions for pernod that I've seen before include "Aperitif flavoured with aniseed" , "Aniseed-flavoured aperitif" , "Drink flavoured with aniseed" , "Brand of pastis" , "Aniseed-flavoured liquor" .)