Unsealed letter to a writer (4)
I believe the answer is:
open
'unsealed' is the definition.
(associated in meaning)
'letter to a writer' is the wordplay.
'letter' becomes 'o' ().
'to a writer' becomes 'pen' (something which writes).
'o'+'pen'='OPEN'
(Other definitions for open that I've seen before include "Plain" , "Not enclosed" , "Traditional room heater" , "Allowing entry and exit" , "Unfasten" .)