Romanian-born pioneer of the theatre of the absurd (7)
I believe the answer is:
ionesco
(Other definitions for ionesco that I've seen before include "Eugene -, Romanian-born French playwright" , "French dramatist, leading figure in the theatre of the absurd, d. 1994" , "O so nice (anag) - playwright" , "Playwright associated with the theatre of the absurd" , "Absurdist playwright" .)