Resolute old king gets one married (4)
I believe the answer is:
grim
'resolute' is the definition.
(I've seen this before)
'old king gets one married' is the wordplay.
'old king' becomes 'GR' (Georgius Rex, Latin for King George).
'gets' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'married' becomes 'm'.
'gr'+'i'+'m'='GRIM'
(Other definitions for grim that I've seen before include "Stern and unsmiling" , "Stern, cruel" , "Serious; dour" , "Dour and severe" , "Dour, forbidding" .)