Riddle cracked in game (6)
I believe the answer is:
enigma
'riddle' is the definition.
(synonyms)
'cracked in game' is the wordplay.
'cracked' is an anagram indicator.
'in'+'game'='ingame'
'ingame' is an anagram of 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Mysterious person/thing" , "Conundrum" , "WWII cipher" , "Mysterious or puzzling thing" , "Puzzle of gamine (6)," .)