Metropolitan Police HQ (8,4)
I believe the answer is:
scotland yard
'metropolitan police hq' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for scotland yard that I've seen before include "London CID" , "London force HQ" , "Detective department of the Metropolitan Police" , "(New) office for the Met" , "Old London police HQ" .)