King John's last to find out (5)
I believe the answer is:
learn
'to find out' is the definition.
('learn' can be a synonym of 'find out')
'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'
(Other definitions for learn that I've seen before include "Commit to memory" , "Ascertain" , "Comprehend" , "Become aware of" , "Acquire information" .)