Drink was unfinished half of bitter (5)
I believe the answer is:
water
'drink' is the definition.
(I know that water is a type of drink)
'was unfinished half of bitter' is the wordplay.
'unfinished' means to remove the last letter.
'half of' indicates taking half.
'was' with its last letter removed is 'wa'.
'bitter' cut in half is 'ter'.
'wa'+'ter'='WATER'
(Other definitions for water that I've seen before include "Basic liquid" , "Quality of diamonds" , "What makes the Somme" , "(Of eyes) produce tears" , "Drink - type of 1 across" .)