Unacceptable to allow opening (6)
I believe the answer is:
outlet
'opening' is the definition.
(I know that outlet is a type of opening)
'unacceptable to allow' is the wordplay.
'unacceptable' becomes 'out' (out can mean incorrect or inappropriate).
'to allow' becomes 'let' ('let' can be a synonym of 'allow').
'out'+'let'='OUTLET'
(Other definitions for outlet that I've seen before include "Point where goods are sold" , "Way of release" , "Retail point" , "River mouth; shop" , "Opening, vent." .)