Exposed old writer (4)
I believe the answer is:
open
'exposed' 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' (I've seen this before).
'o'+'pen'='OPEN'
(Other definitions for open that I've seen before include "Unlock" , "Golf tournament" , "Not enclosed" , "Unrestricted; ready for business" , "Freely accessible" .)