Answer: linchpin

AnswerCrossword Clue
LINCHPINMainstay
LINCHPINCrucial unifying element
LINCHPINKeystone
LINCHPINAxle insert
LINCHPINCritical element
LINCHPINKey element
linchpinA pin passed through the end of an axle to keep a wheel in position
linchpinA person or thing vital to an enterprise or organization
linchpina locking pin inserted in the end of a shaft, also LYNCHPIN
linchpinsomething that holds the various elements of a complicated structure together.