Based on numerical studies of apertured metallic nanocavity structures, we describe a new intra-particle plasmonic interaction pathway that couples the plasmon resonance modes of the aperture edge and the cavity. In contrast to the inter-particle coupling schemes that require precisely arrayed nanoparticles, this intra-particle coupling scheme achieves the tunability in plasmonic resonance wavelength using a single standalone nanostructure. In addition, when the aperture edge is made sharp, it functions dually as a tip that amplifies its near-field producing the local filed enhancement effect. We investigate the details of the coupling mechanism and identify the dominant role of the tip mode in determining the coupling efficiency numerically. The numerical model results in good agreement with recent experimental results. This intra-particle coupling mechanism will help the monolithic integration of plasmonic functionalities and its application for the nanoscale spectroscopy of biological structures in vivo.