A representative of the Sovereign again on board or an assailant? (9)
I believe the answer is:
aggressor
'an assailant?' is the definition.
('aggressor' can be a synonym of 'assailant')
'a representative of the sovereign again on board or' is the wordplay.
'a representative of the sovereign again on board' becomes 'aggress' (I am not sure about this - if you are sure you should believe this answer much more).
'aggress'+'or'='AGGRESSOR'
Can you help me to learn more?
(Other definitions for aggressor that I've seen before include "The one who attacks first" , "The one who starts the fight" , "Hostile attacker" , "He started the fight" , "One attacking without provocation" .)