A container is slightly open (4)
I believe the answer is:
'slightly open' is the definition.
(I've seen this in another clue)
'a container' is the wordplay.
'container' becomes 'jar' (jar is a kind of container).
'is' acts as a link.
(Other definitions for ajar that I've seen before include "Mourn" , "not entirely open" , "opened tentatively?" , "A litle bit open" , "(Door) to, not shut" .)