Old King taking on difficult question (5)
I believe the answer is:
grill
'question' is the definition.
(to grill is to interrogate)
'old king taking on difficult' is the wordplay.
'old king' becomes 'GR' (Georgius Rex, Latin for King George).
'taking on' means one lot of letters go next to another.
'difficult' becomes 'ill' (ill could historically mean difficult).
'gr'+'ill'='GRILL'
(Other definitions for grill that I've seen before include "Interrogate - cook" , "Question hard" , "Question aggressively" , "interrogation?" , "Do well" .)