Almost open a container (4)
I believe the answer is:
ajar
'almost open' is the definition.
(I've seen this in another clue)
'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 "Half open" , "not slammed shut" , "(Door) to, not shut" , "partly opened" , "Unlatched" .)