Forget naughty nun is entertaining king (7)
I believe the answer is:
unlearn
'forget' is the definition.
(to unlearn something is to forget it)
'naughty nun is entertaining king' is the wordplay.
'naughty' indicates an anagram.
'is entertaining' indicates putting letters inside.
'king' becomes 'lear' (King Lear).
'nun' anagrammed gives 'unn'.
'unn' enclosing 'lear' is 'UNLEARN'.
(Other definitions for unlearn that I've seen before include "Deliberately forget" , "Try to forget something" , "Cease to hold (previous teaching)" .)