Welcome, Cynthia. In case people don’t know, ‘Clara Mack’ is an American wisteria, and white-flowered. It is said to be much less rampant than the more common Asian types (Wisteria sinessis or W. floribunda), and blooms at a younger age.

I have not grown it here, but Brooklyn Botanic Garden says about pruning: “it flowers on short, leafy shoots, or pedicels, that arise from buds on the previous year’s wood. To encourage flowering, head back stems after blooms have faded; if necessary, head back a second time in late winter, leaving at least three or four buds. Train shoots to establish a framework or increase vine height; head back once the desired height and width have been achieved. Old plants can be renewed by severe pruning, almost to the ground.”