Playwright had 'is 'n' 'er mixed up (8)
I believe the answer is:
sheridan
'playwright' is the definition.
(I know that Sheridan is a playwright)
'had is n er mixed up' is the wordplay.
'mixed up' indicates anagramming the letters.
'had'+'is'+'n'+'er'='hadisner'
'hadisner' anagrammed gives 'SHERIDAN'.
(Other definitions for sheridan that I've seen before include "Irish playwright and politician, d. 1816" , "Creator of Mrs Malaprop in The Rivals" , "School for Scandal playwright" , "Richard Brinsley . . . . . . . . was a great 18th century Irish playwright" , "Irish dramatist" .)