Answer:

underground

Answer	Crossword Clue
UNDERGROUND	Secret revolutionary force
UNDERGROUND	Clandestine group
UNDERGROUND	Blitz shelter

UNDERGROUND	Escaping detection
UNDERGROUND	London subway.
UNDERGROUND	Maquis, for instance.
UNDERGROUND	British subway system
UNDERGROUND	London transportation
UNDERGROUND	Like Tubman's railroad

Crossword

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