Being sloshed, a mob jeer scout rally (8)
I believe the answer is:
jamboree
'scout rally' is the definition.
(I've seen this before)
'being sloshed a mob jeer' is the wordplay.
'being sloshed' is an anagram indicator.
'a'+'mob'+'jeer'='amobjeer'
'amobjeer' anagrammed gives 'JAMBOREE'.
(Other definitions for jamboree that I've seen before include "beano" , "Rally of Guides or Scouts" , "Boy Scouts' international gathering" , "where lad of 14 [BOY SCOUT] meets more troops?" , "Lively celebration" .)