Understanding old game of chance (4)
I believe the answer is:
keno
'game of chance' is the definition.
(I've seen this before)
'understanding old' is the wordplay.
'understanding' becomes 'ken' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ken'+'o'='KENO'
(Other definitions for keno that I've seen before include "2 8 [CASINO GAME]" , "game of chance" , "gambling game" , "A bingo-like game" , "gambler's choice" .)