They've a will to reject schedule cutting taxes (9)
I believe the answer is:
testators
'they've a will' is the definition.
I can't tell whether this definition defines the answer.
'reject schedule cutting taxes' is the wordplay.
'reject' is a reversal indicator.
'schedule' becomes 'rota' (a rota is a schedule).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'taxes' becomes 'tests' (I can't explain this - if you can you should give a lot more credence to this answer).
'rota' in reverse letter order is 'ator'.
'ator' inserted into 'tests' is 'TESTATORS'.
'to' is the link.
Can you help me to learn more?
(Other definitions for testators that I've seen before include "People who have made wills" , "People willing" .)