To lead the vanguard; to spearhead or take the lead in an advance, campaign, or effort.
彼は新規事業の立ち上げで先陣を切った。
He took the lead in launching the new business.
環境問題の対策で若い世代が先陣を切って行動し始めた。
The younger generation has begun to take the lead in acting on environmental issues.
緊急時には彼女がいつも先陣を切って助けに行く。
In emergencies she always takes the lead and goes to help.