Property tax will keep the gamekeeper busy (6,4)
I believe the answer is:
estate duty
'keep the gamekeeper busy' is the definition.
I can't judge whether this defines the answer.
'property tax' is the wordplay.
'property' becomes 'estate' (estate is a kind of property).
'tax' becomes 'duty' (duty can mean a tax or toll).
'estate'+'duty'='ESTATE DUTY'
'will' is the link.
Can you help me to learn more?
(Other definitions for estate duty that I've seen before include "Passing on charge" , "Dues payable upon a death" , "Inheritance tax" .)