Argued with royal veteran losing crown (8)
I believe the answer is:
reasoned
'argued' is the definition.
('reason' can be a synonym of 'argue')
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for reasoned that I've seen before include "clear" , "sound" , "Logically valid" , "Logically derived" , "Based on logic" .)