UK's Treasury Department (9)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
exchequer
'uk's treasury department' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for exchequer that I've seen before include "Department in charge of a nation's finances" , "royal treasury" , "Purse" , "Government coffers" , "old government office" .)