Drink up, chum (3)
I believe the answer is:
pal
'chum' is the definition.
(both can mean a friend)
'drink up' is the wordplay.
'drink' becomes 'lap' (lap can mean to drink).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'lap' back-to-front is 'PAL'.
(Other definitions for pal that I've seen before include "Buddy, chum" , "Bud" , "Friend (colloq.)" , "China" , "Crony" .)