Former navy department (9)
I believe the answer is:
admiralty
'former navy department' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for admiralty that I've seen before include "Former Department in charge of the navy in Britain" , "HQ" , "Department of 1 across in charge of the Navy" , "UK state department in charge of the Royal Navy" .)