Mystery in game unravelled (6)
I believe the answer is:
enigma
'mystery' is the definition.
('enigma' can be a synonym of 'mystery')
'in game unravelled' is the wordplay.
'unravelled' is an anagram indicator.
'in'+'game'='ingame'
'ingame' with letters rearranged gives 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Puzzle found in gem, Ian" , "WW2 code" , "'Mystery, riddle (6)'" , "Difficult problem" , "closed book" .)