Intimidating behaviour in the past involving historic monarch (5)
I believe the answer is:
aggro
'intimidating behaviour' is the definition.
(short for aggression)
'past involving historic monarch' is the wordplay.
'past' becomes 'ago' (similar in meaning).
'involving' means one lot of letters goes inside another.
'historic monarch' becomes 'GR' (short for Georgius Rex, King George in Latin).
'ago' going around 'gr' is 'AGGRO'.
'in the' is the link.
(Other definitions for aggro that I've seen before include "belligerent behaviour" , "Hostile troublemaking" , "Hostility" , "belligerence" , "offensive behaviour" .)