Partly open for a drink (4)
I believe the answer is:
ajar
'partly open' is the definition.
(as in a door which has been left ajar)
'a drink' is the wordplay.
'drink' becomes 'jar' (colloquial).
'a'+'jar'='AJAR'
'for' acts as a link.
(Other definitions for ajar that I've seen before include "It's nearly open" , "Not quite closed" , "not slammed shut" , "Barely open" , "Partially open" .)