Game in form of a puzzle (6)
I believe the answer is:
enigma
'a puzzle' is the definition.
(I've seen this before)
'game in form' is the wordplay.
'form' indicates anagramming the letters.
'game'+'in'='gamein'
'gamein' with letters rearranged gives 'ENIGMA'.
'of' is the link.
(Other definitions for enigma that I've seen before include "Conundrum" , "Puzzling person/thing" , "'Mystery, riddle (6)'" , "'Puzzle, riddle (6)'" , "Mystery U puzzle" .)