Set straight off as bar's opening is due (8)
I believe the answer is:
disabuse
'set straight' is the definition.
(thesaurus)
'off as bar's opening is due' is the wordplay.
'off' is an anagram indicator.
'opening' suggests taking the first letters.
The initial letter of 'bars' is 'b'.
'as'+'b'+'is'+'due'='asbisdue'
'asbisdue' is an anagram of 'DISABUSE'.
(Other definitions for disabuse that I've seen before include "Free from a false idea" , "Rid (one of wrong idea)" , "Give correction to" , "Rid someone (of a mistaken idea)" , "Enlighten" .)