Open a drink (4)
I believe the answer is:
ajar
'open' is the definition.
(similar in meaning)
'a drink' is the wordplay.
'drink' becomes 'jar' (colloquial).
'a'+'jar'='AJAR'
(Other definitions for ajar that I've seen before include "Partially open" , "Partly open" , "Fractionally open" , "Not quite shut" , "Open by a chink" .)