Affair of honour expected before end of quarrel (4)
I believe the answer is:
duel
'affair of honour' is the definition.
(I've seen this before)
'expected before end of quarrel' is the wordplay.
'expected' becomes 'due' (similar in meaning).
'before' says to put letters next to each other.
'end of' indicates one should take the final letters.
The last letter of 'quarrel' is 'l'.
'due'+'l'='DUEL'
(Other definitions for duel that I've seen before include "honourable affair" , "Prearranged engagement" , "shoot-out?" , "Gentlemen's disagreement" , "Prearranged fight with deadly weapons between two people" .)