Answer:

knuckle

Answer	Crossword Clue
KNUCKLE	Type of sandwich?
KNUCKLE	Get serious, with "down"
KNUCKLE	Type of pitched ball.

KNUCKLE	Kind of ball or sandwich
KNUCKLE	Part of a fist
KNUCKLE	Certain joint
KNUCKLE	Part of a hand

Crossword

a puzzle consisting of a grid of squares and blanks into which words crossing vertically and horizontally are written according to clues.