Poser in game that's fixed (6)
I believe the answer is:
enigma
'poser' is the definition.
(I've seen this in another clue)
'in game that's fixed' is the wordplay.
'that's fixed' indicates anagramming the letters.
'in'+'game'='ingame'
'ingame' anagrammed gives 'ENIGMA'.
(Other definitions for enigma that I've seen before include "WWII cipher" , "Riddle - German coding machine" , "closed book" , "Solution to mystery" , "Title of Elgar's set of variations" .)