Frank; not closed (4)
I believe the answer is:
I believe this clue is a double definition.
'frank' is the first definition.
'not closed' is the second definition.
(I know this)
(Other definitions for open that I've seen before include "Eager to receive" , "Unenclosed" , "simple" , "Undo; ajar" , "Undo; ready for business" .)