Concern for a watcher (7)
I believe the answer is:
lookout
This clue is a double definition.
'concern' is the first definition.
('that's your lookout')
'for a watcher' is the second definition.
(lookout is a kind of watcher)
(Other definitions for lookout that I've seen before include "Be careful!" , "Outlook for sentry" , "Watchman" , "Sentinel, watcher" , "View" .)