Pasta's gone off with container slightly open (4)
I believe the answer is:
ajar
'slightly open' is the definition.
(as in a door left ajar)
'pasta's gone off with container' is the wordplay.
'gone' becomes 'past' (similar in meaning).
'off' suggests deleting specific letters.
'with' says to put letters next to each other.
'container' becomes 'jar' (jar is a kind of container).
'pasta' with 'past' removed is 'a'.
'a'+'jar'='AJAR'
(Other definitions for ajar that I've seen before include "near closing?" , "Partially open" , "Raja (anag.)" , "Half-open" , "Unlatched" .)