Partially open a container (4)
I believe the answer is:
ajar
'partially open' is the definition.
(I've seen this before)
'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 "Raja (anag.)" , "Half open" , "Not quite closed" , "not slammed shut" , "letting in a little light?" .)