A container that's not closed (4)
I believe the answer is:
ajar
'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'
'that's' is the link.
(Other definitions for ajar that I've seen before include "Left open" , "Mourn" , "near closing?" , "not entirely open" , "A litle bit open" .)