Devious game in mystery (6)
I believe the answer is:
enigma
'mystery' is the definition.
('enigma' can be a synonym of 'mystery')
'devious game in' is the wordplay.
'devious' indicates an anagram.
'game'+'in'='gamein'
'gamein' is an anagram of 'ENIGMA'.
(Other definitions for enigma that I've seen before include "German military cipher machine" , "Elgar Variations nickname" , "WW two Ger. code device; puzzle" , "Difficult problem" , "Mysterious puzzle" .)