bell, gong, chimes
鉦 centers on resonant metal percussion instruments: a bell, gong, or set of chimes, all producing a clear ringing sound.
鉦 combines the metal radical 金 with 正, which likely contributes a phonetic element. The character is used for metal percussion instruments.
The left side 金 (metal) and the right side 正 (correct, straight) suggest a metal object that produces a correct, clear tone when struck: a bell or gong.
For セイ, imagine a bell ringing out a clear 'say' sound: say -> セイ, as the metal gong speaks its note.