na-adjective, noun which may take the genitive case particle 'no', noun
essential; crucial; vital; key
Describes the most important or indispensable part of something. Often used in set phrases like 肝心なこと (the crucial thing) or 肝心な時に (at the critical moment).
肝心なのは、最後まであきらめないことだ。
The essential thing is not to give up until the end.
肝心な時に限って、彼はいない。
He's never around at the crucial moment.