Drink up to get a quid (4)
I believe the answer is:
plug
'a quid' is the definition.
('plug' can be a synonym of 'quid')
'drink up' is the wordplay.
'drink' becomes 'gulp' (gulping is a kind of drinking).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'gulp' backwards is 'PLUG'.
'to get' acts as a link.
(Other definitions for plug that I've seen before include "Stopper - publicise" , "Promote" , "It fits into electric socket" , "Stopper - advertisement" , "Piece of publicity" .)