Answer: creepup

AnswerCrossword Clue
CREEPUP

Approach ninja-like

CREEPUP

Approach slowly

CREEPUP

Approach surreptitiously, with "on"

CREEPUP
Approach with stealth
CREEPUP
Rise gradually
CREEPUPApproach sneakily
CREEPUPAdvance little by little.
CREEPUPApproach stealthily