Drink up, mate! (3)
I believe the answer is:
pal
'mate' is the definition.
(I've seen this before)
'drink up' is the wordplay.
'drink' becomes 'lap' (to lap is to drink up).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'lap' reversed gives 'PAL'.
(Other definitions for pal that I've seen before include "Bud" , "Crony" , "Brother" , "Close friend" , "China" .)