One container is slightly open (4)
I believe the answer is:
ajar
'slightly open' is the definition.
(as in a door left ajar)
'one container' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'container' becomes 'jar' (jar is a kind of container).
'a'+'jar'='AJAR'
'is' acts as a link.
(Other definitions for ajar that I've seen before include "A little bit open, like door" , "near closing?" , "Unlatched" , "not slammed shut" , "Half open" .)