Old writer ready to entertain new ideas? (4)
I believe the answer is:
'ready to entertain new ideas?' 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).
(Other definitions for open that I've seen before include "Freely accessible" , "Sports contest" , "Start - frank" , "with no clear favourite" , "'Frank, not secret (4)'" .)