Answer: caltrap

AnswerCrossword Clue
caltrapany of several plants having spiny heads or fruit, as those of the genera Tribulus and Kallstroemia, or the star thistle, Centaurea calcitrapa
caltrapiron ball with four projecting spikes so disposed that when the ball is on the ground one of them always points upward: used to obstruct the passage of cavalry, armored vehicles, etc.
caltrapa spiny plant, also CALTHROP, CALTROP