A container that’s slightly open (4)
I believe the answer is:
ajar
'slightly open' is the definition.
(as in a door left ajar)
'a container' is the wordplay.
'container' becomes 'jar' (jar is a kind of container).
'a'+'jar'='AJAR'
'that's' acts as a link.
(Other definitions for ajar that I've seen before include "(Door) to, not shut" , "Partially open" , "partly opened" , "Barely open" , "near closing?" .)