No more old tax surplus (6)
I believe the answer is:
excess
'surplus' is the definition.
(I know that surplus can be written as excess)
'no more old tax' is the wordplay.
'no more' becomes 'ex' (I've seen this in other clues).
'old tax' becomes 'cess' (I've seen this before. I am not sure about the 'old' bit.).
'ex'+'cess'='EXCESS'
Can you help me to learn more?
(Other definitions for excess that I've seen before include "More than is needed, desired, or required" , "Over and above what's needed" , "Too much, surplus" , "Overabundance" , "Nothing in it, for the Greeks" .)