Old king overwhelmed by past trouble (5)
I believe the answer is:
aggro
'trouble' is the definition.
(I've seen this before)
'old king overwhelmed by past' is the wordplay.
'old king' becomes 'gr' (Georgius Rex, Latin for King George).
'overwhelmed by' indicates putting letters inside (I've seen this in other clues).
'past' becomes 'ago' ('ago' can be similar in meaning to 'past').
'gr' inserted inside 'ago' is 'AGGRO'.
(Other definitions for aggro that I've seen before include "belligerence" , "Problems and difficulties" , "Aggressive, violent behaviour in short" , "Violence - obstructiveness" , "belligerent troublemaking" .)