Formal written work (8)
I believe the answer is:
treatise
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">formal written work</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 treatise that I've seen before include "Learned work on particular subject" , "Iterates in essay" , "Academic composition" , "Learned written work" , "Learned work formally dealing with a subject" .)