Old writer is undone (4)

I believe the answer is:

open

'is undone' is the definition.
('open' can be a synonym of 'undone')

'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 - accessible" , "Undo; exposed to attack" , "Not enclosed" , "Golf tournament" , "Without concealment" .)

I've seen this clue in the Irish Times.

Want a hint initially instead of a full solution? Install my app