Learn Farsi: شاهد مهم است، پس قاضی به دادگاه احضارش کرد. - The witness is important, so the judge summoned him to court.

احضار کردن is a compound verb meaning "to summon." When adding a direct object enclitic like -ash (him/her/it) to a compound verb, it almost always attaches to the non-verb part (the noun or adjective), rather than the helper verb. So we get احضارش کرد instead of attaching it to the end of the whole phrase.

Does the enclitic -ash in احضارش replace the need for the direct object marker را?

Exactly! The enclitic itself acts as the definite direct object, so you do not need را. The suffix has both the "him" and the specific object marking built into it.

Could I say او را احضار کرد instead of using the enclitic?

Yes, using the independent pronoun او (he/she) with the direct object marker را is perfectly correct and means the exact same thing: قاضی او را به دادگاه احضار کرد. Using the enclitic just makes the sentence a bit more natural and compact.

Is the suffix pronounced -ash or -esh in احضارش?

In formal written Farsi, the third-person singular enclitic is -ash. However, in everyday spoken Farsi (and often when reading aloud), it shifts to -esh. So you will most commonly hear احضارش pronounced as "ehzâresh".