But he didn't rule the waves (6)
I believe the answer is:
canute
'but he didn't rule the waves' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for canute that I've seen before include "Danish King of England, 1016-35" , "Old king of England who tried to stop the tide" , "English king who tried to command the waves" , "The king who thought he could hold back the tide" , "Pre-Norman king of England & Denmark" .)