Being challenged he has a second helping (8)
I believe the answer is:
duellist
'being' is the definition.
(duellist is a kind of being)
'challenged he has a second helping' is the wordplay.
I cannot really understand how this works, but
'he' could be 'ellis' and 'ellis' is found in the answer.
'second' could be 'd' and 'd' is located in the answer.
The remaining letters 'ut' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for duellist that I've seen before include "Person engaged in single combat" , "one fighting on a point of honour" , "Combatant in deadly fight between two people" , "Fighter" , "he calls one out" .)