In each essential, a mind game? (5)
I believe the answer is:
chess
'game?' is the definition.
(chess is a kind of game)
'in each essential a mind' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CHESS' is hidden within 'each essential mind'.
(Other definitions for chess that I've seen before include "Musical - board game" , "A game of skill" , "Sixteen-men-a-side game" , "In which two bishops will participate" , "This is played on board" .)