Answer:

shadowed

Answer	Crossword Clue
SHADOWED	Kept tabs on
SHADOWED	Followed stealthily
SHADOWED	Followed secretly.

SHADOWED	Put a tail on
SHADOWED	Tailed
SHADOWED	Surveilled, say

Crossword

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