Bitterness about head of Government is rubbish (5)
I believe the answer is:
bilge
'rubbish' is the definition.
(thesaurus)
'bitterness about head of government' is the wordplay.
'bitterness' becomes 'bile' (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'head of' says to take the initial letters.
The first letter of 'government' is 'g'.
'bile' enclosing 'g' is 'BILGE'.
'is' is the link.
(Other definitions for bilge that I've seen before include "Hogwash" , "Very long film, book etc" , "Dirty water in bottom of boat - drivel - nonsense" , "Lowest internal portion of a ship's hull" , "Lower part of hull; nonsense" .)