U.S. ___ (4)
(Other definitions for open that I've seen before include "still in debate" , "See 12" , "Not closed or secret" , "This championship includes both professional and amateur golfers" , "Allowing entry and exit" .)
'us' is the definition.
(us refers to ourselves)
This is the entire clue.
(Other definitions for ourselves that I've seen before include "Us, reflexively" , "See 16" , "Emphatically us" , "We or us personally" .)
(Other definitions for homo sapiens that I've seen before include "Relative of 1 across" , "Modern humans" , "The one existing species of Man" , "Human species" .)