In the Russian revolution of 1917 were the czars defeated by the communists?

No, they were not. The czar was toppled in the February Revolution of 1917 when the army and police refused to obey his orders to quell rioting and demonstrations against his rule were going on in Petrograd, Moscow and other cities. An interim government called the Provisional Government was created to govern until a Constitutional Assembly could be elected to make a new constitution.

The "communists" (at that time still named Bolsheviks) toppled the Provisional Government in the October Revolution of 1917. By this time, the czar had already been defeated and was no longer in power.

So, to be precise, the czar was not defeated by the communists.