A container is not closed (4)
I believe the answer is:
ajar
'is not closed' is the definition.
(I've seen this before)
'a container' is the wordplay.
'container' becomes 'jar' (jar is a kind of container).
'a'+'jar'='AJAR'
(Other definitions for ajar that I've seen before include "near closing?" , "A litle bit open" , "Half open" , "A little bit open, like door" , "(Of door) slightly open" .)