Candid old writer (4)
I believe the answer is:
open
'candid' is the definition.
(I've seen this before)
'old writer' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'writer' becomes 'pen' (something which writes).
'o'+'pen'='OPEN'
(Other definitions for open that I've seen before include "Frank and unconcealed" , "Honest - frank" , "Not closed; set going" , "Agape" , "Overt" .)