Unsealed circular letter to writer and editor (6)
I believe the answer is:
opened
'unsealed' is the definition.
(similar in meaning)
'circular letter to writer and editor' is the wordplay.
'circular letter' becomes 'o' (a round or circular letter of the alphabet).
'to writer' becomes 'pen' (something which writes).
'and' says to put letters next to each other.
'editor' becomes 'ed' (abbreviation).
'o'+'pen'+'ed'='OPENED'
(Other definitions for opened that I've seen before include "unwrapped" , "Unfastened or moved a barrier" , "Started, a meeting say" , "Began - undone" , "Gave access using key" .)