Writers in Old English having identical magic phrase (4,6)
I believe the answer is:
open sesame
'magic phrase' is the definition.
'writers in old english having identical' is the wordplay.
'writers' becomes 'pens' (I've seen this before).
'in' indicates putting letters inside.
'old english' becomes 'oe'.
'having' means one lot of letters go next to another.
'identical' becomes 'same' ('same' can be similar in meaning to 'identical').
'pens' placed inside 'oe' is 'opense'.
'opense'+'same'='OPEN SESAME'
(Other definitions for open sesame that I've seen before include "a very successful means of achieving a result" , "door-unlocking spell" , "Spell unlocking doors" , "Magical words" , "Magic password" .)