Formal gathering (8)
I believe the answer is:
function
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">formal</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">gathering</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(function is a kind of gathering)</span><br/ ><br/ ></span>
(Other definitions for function that I've seen before include "social gathering" , "Formal event; mathematic expression" , "An official ceremony" , "Formal social event" , "What a thing is used for" .)