Few Republican electors would dare vote for anyone but Trump. Maybe there’s a tiny chance that some who got their position with the help of Republican sponsors that hate Trump might abstain or vote for some other Republican to keep Trump from getting enough votes. They wouldn’t vote for Clinton — that’s fantasy. Even denying Trump the votes is likely fantasy, as it’d cause a schism in the party.
But suppose it happened anyhow. The election goes to the House, where Republican representatives brawl in the aisles. Either they elect Trump (likely, as this only needs 26 of 32 Republican states), or they compromise and give Pence the Presidency in order to preserve the party, or they fail to elect anyone and Pence remains acting President.