Former coloured squares in treasury (9)
I believe the answer is:
exchequer
'treasury' is the definition.
(I know that treasury can be written as exchequer)
'former coloured squares' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'coloured squares' becomes 'chequer' (I can't justify this - if you can you should believe this answer much more).
'ex'+'chequer'='EXCHEQUER'
'in' is the link.
Can you help me to learn more?
(Other definitions for exchequer that I've seen before include "royal treasury" , "government section" , "Bank of England account" , "Government treasury department" , "where the money goes" .)