WAKEFIELD, Mass.--Lt. Gov. John Kerry, nursing a blister from shaking hands in his U.S. Senate race, scrambled yesterday to clear up a question over campaign finance, then fulfilled a non-political promise and took his children to the circus.

His Republican rival, businessman Ray Shanue, spent part of the day stumping on Cape Cod with GOP candidates for Congress and local offices. Shamie then closeted himself with briefing papers to prepare for a face-to-face debate with Kerry on Tuesday, their last encounter before the Nov. 6 election.

Kerry summoned reporters to the sidewalk in front of Boston Garden yesterday to announce he had decided to return $10,000, part of a larger contribution he received from the Democratic Senate Campaign Committee, because that amount was donated by a political action committee.

Kerry, who first challenged the other Senate candidates to wage a "PAC-free" campaign, said he was returning the money because of a "perception" that he was not holding to that standard and because he wanted to focus on "the issues in this campaign."

Earlier in the day, Kerry appeared in Lawrence with Sen. Paul Tsongas, the Democrat who precipitated this race by retiring, then hurried to the West Side Social Club in Wakefield to address a Democratic unity breakfast.

"Let me say what I think is at stake," Kerry told a gathering of nearly 100 party loyalists. "The United States Senate is one of the most important positions in our country."

He attacked Shamie for his opposition to the proposed Equal Rights Amendment and the nuclear weapons freeze and criticized his opponent for lacking the independence to "say no" to President Reagan.

Democratic House Speaker Thomas P. "Tip" O'Neill Jr., who supported U.S. Rep. James Shannon in the Democratic primary, was invited to the breakfast but did not show.

On the PAC issue, Kerry at first insisted that he had no reason to return the $10,000 because it was part of a $372,131 contribution made by the national Democratic group from various sources across the country. Kerry said that fundraising device satisfied the goal of keeping wealthy special interest groups from dominating the political process.

The $10,000 donation was made by Democrats for the '80s, a Washington-based political action committee. The PAC bought tickets to a fund-raiser on Sept. 14, co-chaired by Sen. Edward M. Kennedy '54, to benefit the winner of the Democratic Senate primary.