Government department rebuilt at Surrey (8)
I believe the answer is:
treasury
'government department' is the definition.
(treasury is a kind of government department)
'rebuilt at surrey' is the wordplay.
'rebuilt' indicates anagramming the letters.
'at'+'surrey'='atsurrey'
'atsurrey' with letters rearranged gives 'TREASURY'.
(Other definitions for treasury that I've seen before include "Government finance department" , "The funds of a state" , "Bank" , "Exchequer - store for valuables" , "British Department of Finance" .)