Mystery of Vietnam GI never going back (6)
I believe the answer is:
enigma
'mystery' is the definition.
('enigma' can be a synonym of 'mystery')
'of vietnam gi never going back' is the wordplay.
'of' indicates the answer is hidden within the clue.
'going back' is a reversal indicator.
'amgine' is hidden in the letters of 'vietnam gi never'.
'amgine' reversed gives 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Something mysterious and puzzling" , "Title of Elgar's set of variations" , "Puzzling person/thing" , "Puzzle of gamine (6)," , "Mysterious person/thing" .)