Drink up like a dog, so named (3)
I believe the answer is:
pal
'a dog so named' is the definition.
I can't tell whether this defines the answer.
'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'.
'like' acts as a link.
Can you help me to learn more?
(Other definitions for pal that I've seen before include "Mate, informal friend" , "Friend (colloq.)" , "Mate, buddy" , "Brother" , "Close friend" .)