Passable old writer (4)
I believe the answer is:
open
'passable' is the definition.
('open' can be similar in meaning to 'passable')
'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 "Not closed; set going" , "Undo; ajar" , "Not shut up or hidden" , "Public" , "Widely known" .)