Someone must be willing to appoint him (8)
I believe the answer is:
executor
'appoint him' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'someone must be willing to' is the wordplay.
I cannot really understand how this works, but
'someone' could be 'ex' (ex is a kind of someone) and 'ex' is found within the answer.
'to' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for executor that I've seen before include "Estate manager" , "Administrator" , "One is appointed to carry out the terms of a will" , "Person who sees that a will is followed" , "Man appointed by testator" .)