Surreptitious movement (7)
I believe the answer is:
stealth
'surreptitious movement' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stealth that I've seen before include "Cautious and surreptitious action or movement" , "quality needed by cat burglars" , "Low-visibility military technology" , "Halt set in furtive manner" , "Avoidance of detection by moving carefully" .)