Old queen cuts support for national treasury (9)
I believe the answer is:
exchequer
'national treasury' is the definition.
(I've seen this before)
'old queen cuts support for' is the wordplay.
'old' becomes 'ex'.
'queen' becomes 'qu' (abbreviation for queen).
'cuts' means one lot of letters goes inside another (some letters cut their way into other letters).
'support for' becomes 'cheer' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'qu' inserted within 'cheer' is 'chequer'.
'ex'+'chequer'='EXCHEQUER'
Can you help me to learn more?
(Other definitions for exchequer that I've seen before include "Department in charge of a nation's finances" , "Purse" , "National treasury" , "government section" , "Treasury accounting department" .)