Atomic container slightly open (4)
I believe the answer is:
ajar
'slightly open' is the definition.
(as in a door left ajar)
'atomic container' is the wordplay.
'atomic' becomes 'a' (eg in A-test, atomic test).
'container' becomes 'jar' (jar is a kind of container).
'a'+'jar'='AJAR'
(Other definitions for ajar that I've seen before include "Not quite closed" , "Partially open" , "opened tentatively?" , "Left open" , "Barely open" .)