Game unfinished after leading player stiffens (8)
I believe the answer is:
starches
'stiffens' is the definition.
(both can mean to harden)
'game unfinished after leading player' is the wordplay.
'game' becomes 'chess' (board game).
'unfinished' means to remove the last letter.
'after' says to put letters next to each other.
'leading player' becomes 'star' (I've seen this in another clue).
'chess' with its last letter taken away is 'ches'.
'ches' after 'star' is 'STARCHES'.
(Another definition for starches that I've seen is " Rash sect using stiffening stuffs".)