A duel, two in conflict, illegal (8)
I believe the answer is:
outlawed
'illegal' is the definition.
('outlawed' can be similar in meaning to 'illegal')
'a duel two in conflict' is the wordplay.
'in conflict' indicates an anagram (I've seen this in other clues).
'a'+'duel'+'two'='adueltwo'
'adueltwo' with letters rearranged gives 'OUTLAWED'.
(Other definitions for outlawed that I've seen before include "banned" , "proscribed" .)