Old writer accessible to all (4)
I believe the answer is:
open
'accessible to all' is the definition.
(I've seen this in another clue)
'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 "Ready for business; spread out; manifest" , "With which to offer hearty welcome" , "Undecided" , "Without concealment" , "unresolved" .)