Endanger surrounding area, which is magnificent (8)
I believe the answer is:
imperial
'magnificent' is the definition.
(thesaurus)
'endanger surrounding area' is the wordplay.
'endanger' becomes 'imperil' ('imperil' can be a synonym of 'endanger').
'surrounding' means one lot of letters goes inside another.
'area' becomes 'a' (maths abbreviation).
'imperil' placed around 'a' is 'IMPERIAL'.
'which is' acts as a link.
(Other definitions for imperial that I've seen before include "Mint --, sweet" , "Related to empire" , "I prime Al to describe the rule of great territories" , "Majestic - wine bottle" , "To do with empire or emperor" .)