King in the ancient past retaining fool, mostly on a routine basis (9)
I believe the answer is:
regularly
'king in the ancient past retaining fool mostly' is the definition.
I can't judge whether this defines the answer.
'routine basis' is the wordplay.
'routine' becomes 'regular' (similar in meaning).
'basis' becomes 'ly' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'regular'+'ly'='REGULARLY'
'on a' acts as a link.
Can you help me to learn more?
(Other definitions for regularly that I've seen before include "How shoes will appear in schooners" , "Evenly" , "At fixed intervals" , "Steadily, unvarying" , "Often - at intervals" .)