Opening quarrel? (6)
I believe the answer is:
breach
I believe this clue is a double definition.
'opening' is the first definition.
(I know that breach is a type of opening)
'quarrel?' is the second definition.
(thesaurus)
(Other definitions for breach that I've seen before include "Break, an agreement say" , "Gap made in a fortification" , "transgression" , "Rupture, rift" , "Opening - quarrel" .)