Old king takes in single doomed person (5)
I believe the answer is:
goner
'doomed person' is the definition.
(I've seen this before)
'old king takes in single' is the wordplay.
'old king' becomes 'GR' (Georgius Rex, Latin for King George).
'takes in' indicates putting letters inside.
'single' becomes 'one' (a single thing - only one).
'gr' enclosing 'one' is 'GONER'.
(Other definitions for goner that I've seen before include "without prayer?" , "Doomed person" , "person about to depart" , "One who can't be saved" , "Lost cause" .)