An addition to a Will (7)
I believe the answer is:
codicil
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">an addition to a will</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for codicil that I've seen before include "Addition to a will" , "Will's supplement" , "Modifier to will" , "Addendum to will" , "Portion added on to a will" .)