Playwright scoffs behind enemy's back (5)
I believe the answer is:
yeats
'playwright' is the definition.
(I know that Yeats is a playwright)
'scoffs behind enemy's back' is the wordplay.
'scoffs' becomes 'eats' (I've seen this before).
'behind' means one lot of letters go next to another.
'back' says to take the final letters.
The last letter of 'enemy' is 'y'.
'eats' after 'y' is 'YEATS'.
(Other definitions for yeats that I've seen before include "he wrote" , "W B . . . . ., great Irish poet" , "Irish poet (The Winding Stair)" , "W. B. -- , Irish poet/dramatist" , "Poet William Butler or painter Jack B" .)