Open a container (4)
I believe the answer is:
ajar
'open' is the definition.
(similar in meaning)
'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 "(Of door) slightly open" , "letting in a little light?" , "Part-open" , "partly opened" , "Not quite closed" .)