A container partly open (4)
I believe the answer is:
'partly open' is the definition.
(I've seen this before)
'a container' is the wordplay.
'container' becomes 'jar' (jar is a kind of container**).
(Other definitions for ajar that I've seen before include "not entirely open" , "Part-open" , "partly opened" , "It's nearly open" , "A little bit open, like door" .)