Obvious, not the first tragic old king (4)
I believe the answer is:
lear
'tragic old king' is the definition.
(Shakespeare's King Lear)
'obvious not the first' is the wordplay.
'obvious' becomes 'clear' (similar in meaning).
'not the first' means to remove the first letter.
'clear' with its first letter taken off is 'LEAR'.
(Other definitions for lear that I've seen before include "Tragic Shakespeare king - quite real" , "Eng. poet (The Jumblies)" , "Tragic Shakepearean king, for real" , "See 27 Across" , "Tragic character" .)