Legal power to write it and have the last say! (9)
I believe the answer is:
authority
'say' is the definition.
(thesaurus)
'legal power to write it and have the last' is the wordplay.
I cannot really see how this works, but
'write' could be 'author' (authoring is a kind of writing) and 'author' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present 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 authority that I've seen before include "Power, influence" , "Expert; permission" , "Mandate" , "Power to expercise control" .)