Unacceptable entrances in the open air (8)
I believe the answer is:
outdoors
'in the open air' is the definition.
(I've seen this before)
'unacceptable entrances' is the wordplay.
'unacceptable' becomes 'out' (out can mean incorrect or inappropriate).
'entrances' becomes 'doors' (door is a kind of entrance).
'out'+'doors'='OUTDOORS'
(Other definitions for outdoors that I've seen before include "Al fresco" , "In the open air" , "abroad" , "Is that great?" , "Alfresco" .)