Old security guard, say, from treasury? (9)
I believe the answer is:
exchequer
'treasury?' is the definition.
('exchequer' can be a synonym of 'treasury')
'old security guard say' is the wordplay.
'old' becomes 'ex'.
'security guard say' becomes 'chequer' (I can't explain this - if you can you should believe this answer much more).
'ex'+'chequer'='EXCHEQUER'
'from' is the link.
Can you help me to learn more?
(Other definitions for exchequer that I've seen before include "Government funds" , "Purse" , "Government funding?" , "Department in charge of a nation's finances" , "large supply of money" .)