This study investigates the implementation of non-smooth nonlinear energy sinks, namely with clearance-contacts (CC-NESs), for passive seismic mitigation of a 1D shear frame (primary) structure. To this end, two nonparasitic (in the sense that they do not add extra mass into the structure) local NESs incorporating single-sided clearance contacts are introduced, one at each of the top two stories of the structure. We investigate the nonlinear management of seismic input energy to the primary structure, showcasing the effectiveness of the CC-NESs compared to classical tuned mass dampers (TMDs). Utilizing historical earthquakes scaled for severe excitation, a multi-task optimization approach is employed to determine optimal CC-NESs design parameters, with the main objectives to decrease the input seismic energy and maintain low overall acceleration levels. The results show that the implementation of non-smooth CC-NESs yields significant benefits, exceeding those of TMDs which are linear and narrowband devices. First, due to the action of the CC-NESs a drastic reduction of seismic input energy into the primary structure is achieved compared to that of the corresponding unprotected structure. Second, the CC-NESs facilitate rapid, robust, and irreversible targeted energy transfer (TET) from the primary structure to the local CC-NESs, leading to passive energy confinement and local dissipation. Third, the clearance nonlinearities induce intermodal targeted energy transfer (IMTET) from low to high-frequency structural modes of the primary structure, thereby enhancing energy dissipation performance even during the initial cycles of the structural response, i.e., in the critical highly energetic regime of the seismic response. Remarkably, such enhanced seismic mitigation performance of the CC-NESs is achieved while simultaneously maintaining low acceleration levels. This performance is attributable to the synergy of TET-based NES and IMTET, occurring in the physical and modal spaces, respectively. Lastly, we demonstrate robustness performance of the optimized CC-NESs across a broad spectrum of seismic input energy levels and frequency content, even under severe ground excitations. Further extensions of this methodology to 2D are discussed.