A bit open, but not ultimately ajar (5)
I believe the answer is:
piece
'a bit' is the definition.
('piece' can be a synonym of 'bit')
I don't understand the remainder of the clue.
(Other definitions for piece that I've seen before include "Chessman; portion" , "Fragment" , "man on board" , "Component; portion" , "Bishop, for one" .)