### Clue: Believe head of state's a tool of the Treasury? (8)
### Answer: RECKONER 
 ### Explanation: Definition: a tool of the Treasury
 RECKON(to believe/to have a particular opinion) + ER(abbrev. for "Elizabeth Regina", the Queen, head of state of the UK).
Defn: A table or device to assist with calculations, financial ones in the Treasury.
A table:  , and an ancient tool: