King John's last to find out (5)
I believe the answer is:
learn
'find out' is the definition.
(I know that find out can be written as learn)
'king john's last' is the wordplay.
'king' becomes 'lear' (King Lear).
'last' says to take the final letters.
The final letter of 'john' is 'n'.
'lear'+'n'='LEARN'
'to' acts as a link.
(Other definitions for learn that I've seen before include "Commit to memory" , "Ascertain" , "Comprehend" , "Become aware of" , "Acquire information" .)