Writers in Old English with identical magic words (4,6)
I believe the answer is:
open sesame
'magic words' is the definition.
'writers in old english with identical' is the wordplay.
'writers' becomes 'pens' (I've seen this before).
'in' indicates putting letters inside.
'old english' becomes 'oe'.
'with' is a charade indicator (letters next to each other).
'identical' becomes 'same' ('same' can be similar in meaning to 'identical').
'pens' placed into 'oe' is 'opense'.
'opense'+'same'='OPEN SESAME'
(Other definitions for open sesame that I've seen before include "Words of Ali Baba" , "a very successful means of achieving a result" , "free entry" , "Spell unlocking doors" , "spelling words?" .)