Playwright I harm (6)
I believe the answer is:
oneill
'playwright' is the definition.
(Eugene O'Neill, American playwright 1888-1953)
'i harm' is the wordplay.
'i' becomes 'one' (Roman numeral for one).
'harm' becomes 'ill' (I've seen this in another clue).
'one'+'ill'='ONEILL'
(Other definitions for oneill that I've seen before include "Eugene --, US dramatist" , "Eugene -, US playwright (The Iceman Cometh)" , "Eugene ..., Tip..., Ryan ... (1'5)" , "American dramatist" .)