Watchman, beware!
I believe the answer is:
lookout
This clue is a double definition.
'watchman' is the first definition.
(lookout is a kind of watchman)
'beware' is the second definition.
(thesaurus)
(Other definitions for lookout that I've seen before include "One who keeps watch, a sentry" , "A sentry or guard, must be watchful" , "Observatory" , "Person keeping watch" , "Guard on duty" .)