Essay about old play such as King Lear (7)
I believe the answer is:
tragedy
'play such as king lear' is the definition.
'essay about old' is the wordplay.
'essay' becomes 'try' (synonyms).
'about' means one lot of letters goes inside another.
'old' becomes 'aged' (similar in meaning).
'try' enclosing 'aged' is 'TRAGEDY'.
(Other definitions for tragedy that I've seen before include "Serious catastrophe" , "story ending unhappily" , "King Lear, perhaps" , "play by Aeschylus?" , "Drama genre" .)