Mystery ingredient of Seagram gin -- enjoyment's up! (6)
I believe the answer is:
enigma
'mystery' is the definition.
('enigma' can be a synonym of 'mystery')
'ingredient of seagram gin enjoyment's up' is the wordplay.
'ingredient of' indicates a hidden word.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'amgine' can be found hidden inside 'seagram gin enjoyments'.
'amgine' backwards is 'ENIGMA'.
(Other definitions for enigma that I've seen before include "closed book" , "Gamine being a bit of a puzzle" , "Mysterious puzzle" , "Mystery U puzzle" , "WW two Ger. code device; puzzle" .)