Game for a bonehead, being drunk (6)
I believe the answer is:
blotto
'being drunk' is the definition.
(slang term)
'game for a bonehead' is the wordplay.
'game' becomes 'lotto' (the UK National Lottery).
'for' means one lot of letters go next to another (I've seen this in other clues).
'a bonehead' becomes 'b' (head letter of 'bone').
'lotto' put after 'b' is 'BLOTTO'.
(Other definitions for blotto that I've seen before include "To bolt away in drunk state" , "Unconscious from drink" , "Tight" , "under the table" , "More than happy" .)