Aggressive call by second in line upset regent (11)
I believe the answer is:
belligerent
'aggressive' is the definition.
('belligerent' can be a synonym of 'aggressive')
'call by second in line upset regent' is the wordplay.
'call' becomes 'bell' (I've seen this before).
'by second in line' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'upset' indicates an anagram.
'regent' with letters rearranged gives 'gerent'.
'bell'+'i'+'gerent'='BELLIGERENT'
Can you help me to learn more?
(Other definitions for belligerent that I've seen before include "hawk" , "Combative" , "Hostile, aggressive" , "fighting" , "Warlike" .)