Old auditor perhaps on call in Treasury department (9)
I believe the answer is:
exchequer
'call in treasury department' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'old auditor perhaps on' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'auditor perhaps' becomes 'chequer' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another.
'ex'+'chequer'='EXCHEQUER'
Can you help me to learn more?
(Other definitions for exchequer that I've seen before include "Purse" , "old government office" , "Treasury accounting department" , "Government funds" , "large supply of money" .)