Not to drink after a . . . (4)
I believe the answer is:
ajar
'not to' is the definition.
(I've seen this before)
'drink after a' is the wordplay.
'drink' becomes 'jar' (colloquial).
'after' means one lot of letters go next to another.
'jar' put after 'a' is 'AJAR'.
(Other definitions for ajar that I've seen before include "Open by a chink" , "A little bit open, like door" , "Raja (anag.)" , "Barely open" , "near closing?" .)