Answer: creepup

AnswerCrossword Clue
CREEPUPApproach ninja-like
CREEPUPApproach slowly
CREEPUPApproach surreptitiously, with "on"
CREEPUPApproach with stealth
CREEPUPRise gradually
CREEPUPApproach sneakily
CREEPUPAdvance little by little.
CREEPUPApproach stealthily