Drink party organiser curtailed - heavens (6)
I believe the answer is:
whisky
'drink' is the definition.
(I know that whisky is a type of John Barleycorn)
'party organiser curtailed heavens' is the wordplay.
'party organiser' becomes 'whip' ('whips' keep discipline in parliamentary parties).
'curtailed' means to remove the last letter.
'heavens' becomes 'sky' (sky is a kind of heavens).
'whip' with its final letter removed is 'whi'.
'whi'+'sky'='WHISKY'
(Other definitions for whisky that I've seen before include "Scotch" , "W in radio alphabet" , "Spirit distilled from malted grain" , "Scottish spirit" , "reportedly available for Thin Lizzy gig?" .)