Most of washington probably thinks that William P. Clark is deputy secretary of state. He was confirmed for the job nearly three weeks ago and has been busily issuing statements and decisions as the second-ranking official in the State Department.

Most of Washington would be wrong. Clark actually is still a California Supreme Court justice.

Clark is still wearing a black robe over his striped pants because decisions of California's highest court do not become effective until the day they are filed, and on that day only the votes of sitting justices are counted.

Clark said yesterday that "in fiarness to the litigants and their attorneys," he did not want to relinquish his seat on the bench until all cases arguede and resolved during his tenure had been filed, least the cases be forced back into the court for reargument.

He offered the "hypothetical" case of a 4-to-3 ruling of the seven-member court, with Clark in the majority. If he leaves the court before the case is filed, the decision would be a tie, and the case might be returned to the court, Clark said.

Clark said he had written his opinions on all of the outstanding cases before accepting the State Department job from his old friend, President Reagan. "In the past 30 days, have not had occasion to write an opinion," he said. "Everything I have to write has been written."

But here is another hypothetical situation: Clark is a conservative voice on the California court, and the state's conservatives have expressed concern that Gov. Edmund G. (Jerry) Brown Jr. might replace him with a liberal. If that happened, a case returned to the court might well result in a different ruling.

Richard Morris, a senior attorney for the court, insisted that Clarkhs desire to see the cases through has nothing to do with ideology. Morris would not identify any of the cases beyond saying there are "less than 10" of them, and he declined to speculate on when Clark might be free to take the oath at state.

Clark said he believed there are two cases remaining and that his vote might not be determinative on either of them. Thus, he said, he might be ready for his swearing-in later this week.

The State Department says Clark's swearing-in has been delayed because "he's having to straighten out a few of his personal things."

The situation is not so unusual in Washington, accounting to a State Department spokesman who drew an analogy between Clark's dilemma and that of a businessman forced to divest himself of certain investments before take a federal job. "People don't drop what they're to take up government serive, a spokesman said.

The department confirmed that, for all intents and purposes, Clark is on the job in Washington and "performing like the deputy secretary," with one exception.

Because Clark does not have signatory powers, the department's third-ranking official, undersecretary for political affairs Walter J. Stoessel Jr., has to take charge when Secretary of State Alexander M. Haig Jr. is out of town.

Despite his temporary fencestraddling between jobs on two coasts, "I'm not holding two jobs as far as salary or anything," Clark said. He is being paid as a California justice -- at a rate of about $72,000 a year -- and is not drawing his federal salary, which will be $50,112 a year.

Officials in California are slightly less sanguine about that end of the arrangement.

"I think Justice Clark must be fair to the California taxpayer," said Gray Davis, Gov. Brown's chief of staff. "If he is primarily working on Supreme Court business, it's fair that California should continue to shoulder the burden of his salary. If he is primarily working on State Department business, it is not fair."

Davis says he is in no position to know which job Clark is concentrating on, but "I'm told he is spending most of his time in Washington, which leads me to believe he is concentrating on the State Department."

Clark confirms that. "My full effort has been here," he said. But he does have to fly back to California "every week or 10 days" to sign court documents because he has no jurisdiction outside his home state. The cost of those trips has come out of his own pocket, he said, except for one trip a couple of weeks ago that he piggy-backed onto State Department business on the West Coast.