The person who missed a joke should not have to become the next one.

A room laughs, one person did not follow, and the quickest way past the gap is to make their confusion the next thing everyone enjoys. It is a small unkindness that never looks like one, and it teaches that person to stop asking. Missing a reference costs nothing. Becoming the material costs the rest of the evening.

When someone did not catch it, explain plainly and briefly, without the flourish that turns the explanation into a second joke. Then carry on as though nothing needed rescuing.