Do drink (5)
I believe the answer is:
tonic
I believe this clue is a double definition.
'do' is the first definition.
(musical tonic in the do-re-mi scale)
'drink' is the second definition.
(tonic is a kind of drink)
(Other definitions for tonic that I've seen before include "something invigorating" , "Small drink of spirits" , "Producing tension of muscles" , "__water; gin mixer" , "shot in the arm" .)