It's obvious Charlie's a foolish old king (5)
I believe the answer is:
clear
'it's obvious' is the definition.
(similar in meaning)
'charlie's a foolish old king' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet).
'a foolish old king' becomes 'lear' (I can't justify this - if you can you should believe this answer much more).
'c'+'lear'='CLEAR'
Can you help me to learn more?
(Other definitions for clear that I've seen before include "bound over" , "Open; not cloudy" , "Fine" , "Empty - vault" , "Lucid, transparent" .)