Open a glass container (4)
I believe the answer is:
ajar
'open' is the definition.
(similar in meaning)
'a glass container' is the wordplay.
'glass container' becomes 'jar' (I've seen this in another clue).
'a'+'jar'='AJAR'
(Other definitions for ajar that I've seen before include "Part-open" , "near closing?" , "Raja (anag.)" , "Unlatched" , "letting in a little light?" .)