Discover King John's end (5)
I believe the answer is:
learn
'discover' is the definition.
('learn' can be a synonym of 'discover')
'king john's end' is the wordplay.
'king' becomes 'lear' (I've seen this before).
'end' indicates one should take the final letters.
The final letter of 'john' is 'n'.
'lear'+'n'='LEARN'
(Other definitions for learn that I've seen before include "Read up on" , "Take in" , "Commit to memory" , "Become aware of" , "Discover - study" .)