King's comeback in Open (4)
I believe the answer is:
ajar
'open' is the definition.
('ajar' can be similar in meaning to 'open')
'king's comeback' is the wordplay.
'king' becomes 'raja' (historical term for an Indian king or prince).
'comeback' shows that the letters should be reversed in order.
'raja' back-to-front is 'AJAR'.
'in' acts as a link.
(Other definitions for ajar that I've seen before include "Not to answer" , "Fractionally open" , "Half-open" , "Not quite closed" , "Raja (anag.)" .)