A vessel not properly closed (4)
I believe the answer is:
ajar
'not properly closed' is the definition.
'a vessel' is the wordplay.
'vessel' becomes 'jar' (jar is a kind of vessel**).
'a'+'jar'='AJAR'
(Other definitions for ajar that I've seen before include "(Door) to, not shut" , "Not to answer" , "A little bit open, like door" , "partly opened" , "Half-closed" .)