Answer:

linchpin

Answer	Crossword Clue
LINCHPIN	Mainstay
LINCHPIN	Crucial unifying element
LINCHPIN	Keystone

LINCHPIN	Axle insert
LINCHPIN	Critical element
LINCHPIN	Key element

Crossword

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