Playwright's agreed a firm rise (6)
I believe the answer is:
ocasey
'playwright's' is the definition.
(Sen O'Casey, 1880-1964)
'agreed a firm rise' is the wordplay.
'agreed' becomes 'yes' (I've seen this before).
'firm' becomes 'co' (a firm is a company).
'rise' shows that the letters should be reversed in order.
'yes'+'a'+'co'='yesaco'
'yesaco' reversed gives 'OCASEY'.
(Other definitions for ocasey that I've seen before include "Sean . . . . . . wrote 'Juno and the Paycock'" , "Sean ..., author of 'The Plough and the Stars'" , "Sean .'..... wrote ` `The Shadow of a Gunman' ' (1'5)" , "Sean .' ....., author of ` `The Silver Tassie' ' (1'5)" , "one connected with The Abbey" .)