Something worn by an infant or marathon runner

BIB

It’s a small, often cloth piece that catches drips or displays a race number.

This clue plays on two different meanings of "bib." For an infant, a bib is a small cloth tied around the neck to catch drool or food. Marathon runners also wear bibs—the numbered tags pinned to their shirts for identification. The word fits both scenarios neatly, making it a clever double-meaning clue.