Terrible danger, with political activists at Buckingham House event (6,5)
I believe the answer is:
garden party
'buckingham house event' is the definition.
(Buckingham Palace often hosts garden parties)
'terrible danger with political activists' is the wordplay.
'terrible' indicates an anagram.
'with' means one lot of letters go next to another.
'political activists' becomes 'party' (a political party).
'danger' anagrammed gives 'garden'.
'garden'+'party'='GARDEN PARTY'
'at' is the link.
(Other definitions for garden party that I've seen before include "Social occasion at Buckingham Palace, perhaps" , "Buckingham Palace event" , "Alfresco social event" , "Formal festive outdoor occasion" , "Lawn event" .)