Surreptitious action (7)
I believe the answer is:
stealth
'surreptitious action' 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" , "Unseen machination" , "By which taxes go up" , "Slyness" , "The last (anag.)" .)