Real mad king! (4)
I believe the answer is:
lear
'king' is the definition.
(King Lear)
'real mad' is the wordplay.
'mad' is an anagram indicator.
'real' is an anagram of 'LEAR'.
(Other definitions for lear that I've seen before include "Humorist, Edward --" , "King -, Edward -" , "Tragic Shakespeare king - quite real" , "Shakespeare's tragic king (4)" , "Tragic Shakespearean king" .)