Something inexplicable in game that's complex (6)
I believe the answer is:
enigma
'something inexplicable' is the definition.
(I've seen this in another clue)
'in game that's complex' is the wordplay.
'that's complex' is an anagram indicator.
'in'+'game'='ingame'
'ingame' with letters rearranged gives 'ENIGMA'.
(Other definitions for enigma that I've seen before include "German military cipher machine" , "Riddle - German coding machine" , "Title of Elgar's set of variations" , "Coding device" , "Mysterious or puzzling thing" .)