Determination? Old King has it (4)
I believe the answer is:
grit
'determination?' is the definition.
(grit can mean determination or tenacity)
'old king has it' is the wordplay.
'old king' becomes 'GR' (Georgius Rex, Latin for King George).
'has' is a charade indicator (letters next to each other).
'gr'+'it'='GRIT'
(Other definitions for grit that I've seen before include "Kind of courage or dirt" , "lamb" , "Courage, determination" , "Fortitude" , "True determination?" .)