May be handy if you're about to pass out (4)
I believe the answer is:
door
'out' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for door that I've seen before include "See 1" , "Swinging barrier" , "Usual entrance" , "Opener" , "Barrier to open and close" .)