Playwright was upset about ending of Macbeth (4)
I believe the answer is:
shaw
'playwright' is the definition.
(George Bernard Shaw)
'was upset about ending of macbeth' is the wordplay.
'upset' says the letters should be written backwards.
'about' indicates putting letters inside.
'ending of' indicates one should take the final letters.
The final letter of 'macbeth' is 'h'.
'was' backwards is 'saw'.
'saw' going around 'h' is 'SHAW'.
(Other definitions for shaw that I've seen before include "Playwright; small wood" , "George Bernard -, Irish dramatist" , "making an appearance in Craiglockhart?" , "writer of plays and critical commentaries" , "Major Barbara playwright" .)