Frank, old writer (4)
I believe the answer is:
open
'frank' is the definition.
('open' can be a synonym of 'frank')
'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 "Frank and unlocked" , "Widely known" , "Frank; unfold" , "Available for business, exposed" , "Honest - frank" .)