Arnold has become Olivers comrade-in-arms (6)
I believe the answer is:
roland
'become olivers comrade-in-arms' is the definition.
I can't tell whether this definition defines the answer.
'arnold' is the wordplay.
'arnold' can be anagrammed to 'ROLAND'.
However, I'm not sure how the anagram is indicated.
'has' acts as a link.
Can you help me to learn more?
(Other definitions for roland that I've seen before include "Paladin" , "Legendary fighter" , "Comrade of Oliver" , "Hero of early French epic poem" , "Legendary French hero" .)