500 or so confused entrances (5)
I believe the answer is:
doors
'entrances' is the definition.
(door is a kind of entrance)
'500 or so confused' is the wordplay.
'500' becomes 'd' ('d' can be a synonym of '500').
'confused' indicates anagramming the letters.
'or'+'so'='orso'
'orso' anagrammed gives 'oors'.
'd'+'oors'='DOORS'
(Other definitions for doors that I've seen before include "Portals" , "Swinging or sliding barriers" , "Openings" , "Barriers to entrances" , "Ways in and out" .)