A glass container is not properly closed (4)
I believe the answer is:
ajar
'not properly closed' is the definition.
'a glass container' is the wordplay.
'glass container' becomes 'jar' (I've seen this in another clue).
'a'+'jar'='AJAR'
'is' is the link.
(Other definitions for ajar that I've seen before include "Open by a chink" , "Not quite shut" , "Fractionally open" , "Raja (anag.)" , "near closing?" .)