Answer: tailing

AnswerCrossword Clue
TAILING

Following behind

TAILING

Close behind

TAILING

Not first

TAILING
Some detective work
TAILING
Following
TAILINGFollowing a suspect.
TAILINGWork for a gumshoe.
TAILINGPrivate eye business.
TAILINGWork for a private eye.
TAILINGPrivate eye's activity
TAILINGOn the heels of