Puzzle in game to be worked out (6)
I believe the answer is:
enigma
'puzzle' is the definition.
(thesaurus)
'in game to be worked out' is the wordplay.
'to be worked out' indicates anagramming the letters.
'in'+'game'='ingame'
'ingame' anagrammed gives 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Mysterious puzzle" , "Difficult problem" , "Puzzle of gamine (6)," , "Gamine being a bit of a puzzle" , "Mysteriously puzzling person or thing" .)