Good party - but sober! (5)
I believe the answer is:
grave
'sober' is the definition.
('grave' can be a synonym of 'sober')
'good party' is the wordplay.
'good' becomes 'g' (abbreviation).
'party' becomes 'rave' (I've seen this before).
'g'+'rave'='GRAVE'
'but' is the link.
(Other definitions for grave that I've seen before include "In which to lay a corpse" , "Serious in cemetery?" , "French accent; very serious" , "Solemn; tomb" , "Grievous, serious" .)