Got to know King Cnut: sea, you heard, must withdraw (6)
I believe the answer is:
learnt
'got to know' is the definition.
(to learn something is to get to know it)
'king cnut sea you heard must withdraw' is the wordplay.
I cannot really understand how this works, but
'king' could be 'lear' (King Lear) and 'lear' is found in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for learnt that I've seen before include "Heard" , "Found out" , "Acquired knowledge or skill" , "Studied" , "Gained knowledge or skill" .)