In retro pubs it's a popular liqueur (6)
I believe the answer is:
pastis
'liqueur' is the definition.
(I know that pastis is a type of liqueur)
'in retro pubs it's a popular' is the wordplay.
'retro' shows that the letters should be reversed in order (retro is Latin for 'backwards').
'in' says the answer is hidden in the clue.
'sitsap' is hidden in the letters of 'pubs its a popular'.
'sitsap' backwards is 'PASTIS'.
(Other definitions for pastis that I've seen before include "Aniseed aperitif" , "Aniseed-flavour aperitif" , "Rectangular" , "Aniseed-flavoured aperitif" , "Alcoholic drink flavoured with aniseed" .)