A sound, if cheap, drink (5)
I believe the answer is:
plonk
I believe this clue is a double definition.
'a sound' is the first definition.
(plonk is a kind of sound)
'cheap drink' is the second definition.
(I've seen this before)
'if' is the link.
(Other definitions for plonk that I've seen before include "Put down noisily" , "cheap drink" , "Drop" , "Put down - cheap wine" , "Ordinary wine" .)