Owed pounds in honourable affair (4)
I believe the answer is:
'honourable affair' is the definition.
(I've seen this before)
'owed pounds' is the wordplay.
'owed' becomes 'due' ('due' can be a synonym of 'owed').
'pounds' becomes 'l' (L can represent the monetary pound).
'in' acts as a link.
(Other definitions for duel that I've seen before include "Fight over a point of honour" , "Fight between two people over an affair of honour" , "One got satisfaction from this" , "Combat to settle a row" , "Single combat" .)