That guy gulped down head of draught bitter (6)
I believe the answer is:
heated
'bitter' is the definition.
(thesaurus)
'that guy gulped down head of draught' is the wordplay.
'that guy' becomes 'he'.
'gulped down' becomes 'ate' (I've seen this in another clue).
'head of' says to take the initial letters.
The initial letter of 'draught' is 'd'.
'he'+'ate'+'d'='HEATED'
(Other definitions for heated that I've seen before include "Raised the temperature" , "Of discussion, angry and hostile" , "Fired-up" , "Vehement" , "Angry or warmed" .)