Drink up, boys, that's fine! (5)
I believe the answer is:
regal
'that's fine' is the definition.
(thesaurus)
'drink up' is the wordplay.
'drink' becomes 'lager' (lager is a kind of drink).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'lager' in reverse letter order is 'REGAL'.
'boys' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for regal that I've seen before include "Grand and imposing" , "King's" , "Fit for a king" , "Portable organ" , "Like a queen?" .)