N.F.L. ball carriers, for short

RBS

They’re often seen sprinting behind the offensive line in football highlights.

This clue is asking for the short form of the players who carry the football in the N.F.L.—running backs. "For short" signals the answer is an abbreviation, and "RBS" stands for "running backs," the position responsible for rushing the ball. It’s a straightforward sports reference with no wordplay, just a common football term.