Seething angry, depart social gathering (6,5)
I believe the answer is:
garden party
'social gathering' is the definition.
(garden party is a kind of social gathering)
'seething angry depart' is the wordplay.
'seething' indicates an anagram (I've seen this in other clues).
'angry'+'depart'='angrydepart'
'angrydepart' is an anagram of 'GARDEN PARTY'.
(Other definitions for garden party that I've seen before include "Social occasion at Buckingham Palace, perhaps" , "Formal festive outdoor occasion" , "Buckingham Palace event" , "Lawn event" , "Alfresco social event" .)