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" .)

I've seen this clue in The Times.

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