Greek plays, others in Old English, I start to adapt (8)
I believe the answer is:
oresteia
'greek plays' is the definition.
(I've seen this before)
'others in old english i start to adapt' is the wordplay.
'others' becomes 'rest' (I've seen this before).
'in' indicates putting letters inside.
'old english' becomes 'oe'.
'start to' indicates taking the first letters.
The first letter of 'adapt' is 'a'.
'rest' going within 'oe' is 'oreste'.
'oreste'+'i'+'a'='ORESTEIA'
(Other definitions for oresteia that I've seen before include "Play trilogy by Aeschylus, see a riot (anag.)" , "Greek plays" , "Aeschylus's play trilogy" , "three tragedies for Aeschylus" .)