Government department’s former auditor mentioned in speech (9)
I believe the answer is:
exchequer
'government department's' is the definition.
(I've seen this before)
'former auditor mentioned in speech' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'auditor' becomes 'checker'.
'mentioned in speech' shows a homophone (sound like).
'checker' sounds like 'chequer'.
'ex'+'chequer'='EXCHEQUER'
(Other definitions for exchequer that I've seen before include "Government coffers" , "royal treasury" , "where the money goes" , "Department in charge of a nation's finances" , "Government funds" .)