Fighter with various duties crossing lines (8)
I believe the answer is:
'fighter' is the definition.
(I've seen this before)
'various duties crossing lines' is the wordplay.
'various' indicates an anagram.
'crossing' is an insertion indicator.
'lines' becomes 'll' (an abbreviation for line written twice).
'duties' anagrammed gives 'dueist'.
'dueist' placed around 'll' is 'DUELLIST'.
'with' acts as a link.
(Other definitions for duellist that I've seen before include "one fighting on a point of honour" , "Fighter" , "Combatant in deadly fight between two people" , "Person engaged in single combat" , "he calls one out" .)