Justification for affair? (8)
I believe the answer is:
occasion
This clue is a double definition.
'justification' is the first definition.
(thesaurus)
'for affair?' is the second definition.
('occasion' can be a synonym of 'affair')
(Other definitions for occasion that I've seen before include "call" , "Provoke" , "Need" , "instance" , "Special event or celebration" .)