A wide-mouthed container, slightly open (4)
I believe the answer is:
ajar
'slightly open' is the definition.
(as in a door left ajar)
'a wide-mouthed container' is the wordplay.
'wide-mouthed container' becomes 'jar' (I've seen this before).
'a'+'jar'='AJAR'
(Other definitions for ajar that I've seen before include "(Of door) slightly open" , "Open by a chink" , "Half-open" , "Fractionally open" , "near closing?" .)