polish
鎣 centers on making metal bright and smooth through rubbing: polishing. The meaning is specific to burnishing metal surfaces.
鎣 combines 金 (metal) with a phonetic component historically related to 熒 (a flickering flame). The upper part suggests a bright, shining quality, fitting the idea of polishing metal to a gleam.
The bottom is 金 (metal), and the top looks like a flame 火 under a cover 冖, as if heating metal to make it shine. Picture polishing a metal piece until it glows like fire.
For エイ, imagine polishing a metal 'A' letter until it gleams: A -> エイ, and the shiny metal reflects the sound.