(n.) One who guards.

airhead armed guard avant-garde bank guard battle line beachhead bridgehead coast guard cordon cordon sanitaire farthest outpost first line forefront front front line front rank front-runner garrison goalie goalkeeper goaltender guard guarder guardsman inlying picket innovation jailer latest fad latest fashion latest wrinkle line new look newfangled device novelty outguard outpost picket pioneer point precursor railhead rear guard scout security guard spearhead the in thing the last word the latest thing train guard van vanguard warder