Game in the form of a puzzle (6)
I believe the answer is:
enigma
'a puzzle' is the definition.
(thesaurus)
'game in the form' is the wordplay.
'the form' indicates an anagram.
'game'+'in'='gamein'
'gamein' is an anagram of 'ENIGMA'.
'of' is the link.
(Other definitions for enigma that I've seen before include "Riddle - German coding machine" , "Puzzle found in gem, Ian" , "WWII cipher" , "Coding device" , "Puzzle for gaming" .)