A mystery to some playing in game (6)
I believe the answer is:
enigma
'a mystery' is the definition.
('enigma' can be a synonym of 'mystery')
'some playing in game' is the wordplay.
'some playing' indicates anagramming the letters.
'in'+'game'='ingame'
'ingame' with letters rearranged gives 'ENIGMA'.
'to' acts as a link.
(Other definitions for enigma that I've seen before include "Puzzle found in gem, Ian" , "Solution to mystery" , "Title of Elgar's set of variations" , "cryptic puzzle!" , "Mysterious person/thing" .)