Very drunk bishop knocked out — it’s all down to chance (5)
I believe the answer is:
lotto
'it's all down to chance' is the definition.
'very drunk bishop knocked out' is the wordplay.
'very drunk' becomes 'blotto' (I've seen this before).
'bishop' becomes 'b' (abbreviation used in chess).
'knocked out' suggests deleting specific letters.
'blotto' with 'b' taken away is 'LOTTO'.
(Other definitions for lotto that I've seen before include "Bingo-like game" , "National gambling game" , "chancy business" , "Game of chance involving balls" , "Children's game; lottery" .)