Mystery morning drink European served up (6)
I believe the answer is:
enigma
'mystery' is the definition.
(synonyms)
'morning drink european served up' is the wordplay.
'morning' becomes 'am' (as in gi:00 AM).
'drink' becomes 'gin' (gin is an alcoholic drink).
'european' becomes 'e' (abbreviation e.g. EU).
'served up' says the letters should be written in reverse (in a down clue, going upwards).
'am'+'gin'+'e'='amgine'
'amgine' reversed gives 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Puzzle of gamine (6)," , "Coding device" , "'Puzzle, riddle (6)'" , "closed book" , "cryptic puzzle!" .)