Close observation (5)
'close observation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for surveillance that I've seen before include "Covert observation" , "Close observation" , "Watch" .)
(Other definitions for watch that I've seen before include "See 21" , "Observe - guard" , "period of duty" , "Timepiece - look out" , "Keep an eye on - timepiece" .)