Treasury's former auditor picked up (9)
I believe the answer is:
exchequer
'treasury's' is the definition.
(synonyms)
'former auditor picked up' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'auditor' becomes 'checker'.
'picked up' shows a homophone (sound like).
'checker' sounds like 'chequer'.
'ex'+'chequer'='EXCHEQUER'
(Other definitions for exchequer that I've seen before include "Department in charge of a nation's finances" , "Treasury accounting department" , "where the money goes" , "old government office" , "Bank of England account" .)