Even with Leonard, I think Clark is an ideal free agent target. I suspect he'll have to take a low dollar contract, perhaps as a partial guarantee.
He's a true combo forward and guarded 4 positions last season, although he'll typically guards 3 or 4. In some games—against the Thunder and Durant, for example—he looked like a defensive stopper in the making.
Leonard has far more offensive potential, but Leonard/Clark would give the Spurs an effective pair of defenders to throw at LeBron/Durant/Kobe/Dirk/etc.