Game in crackers is a mystery (6)
I believe the answer is:
enigma
'mystery' is the definition.
('enigma' can be a synonym of 'mystery')
'game in crackers' is the wordplay.
'crackers' indicates an anagram (informal term for mad).
'game'+'in'='gamein'
'gamein' with letters rearranged gives 'ENIGMA'.
'is a' is the link.
(Other definitions for enigma that I've seen before include "Mysteriously puzzling person or thing" , "Difficult problem" , "'Puzzle, riddle (6)'" , "Mystery U puzzle" , "Puzzle found in gem, Ian" .)