Partly open a container (4)
I believe the answer is:
ajar
'partly open' is the definition.
(as in a door which has been left ajar)
'a container' is the wordplay.
'container' becomes 'jar' (jar is a kind of container).
'a'+'jar'='AJAR'
(Other definitions for ajar that I've seen before include "Unlatched" , "Mourn" , "Open by a chink" , "Not quite closed" , "A little bit open, like door" .)