A clean sheet in soccer betting is a term that is called a shut out as well.
When one team wins the game to nil, or without allowing the other team to score, it is said that the goalkeeper and defense have kept a clean scoresheet for the scorekeeper.
An example of a recent clean sheet betting came in the recent UEFA Euro 2020 qualification when Italy hosted Finland. On the 1×2 money line, Italy was -400, the draw was +450, while Finland was +1500.
But you could also bet that Italy and/or Finland would have held the other team scoreless. Italy to win the game to nil (clean sheet) was yes -162, no +120, while Finland was yes +1200, no -10000.