Keynote (5)
I believe the answer is:
theme
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">keynote</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for theme that I've seen before include "tune used in Enigma Variations?" , "Recurring melody" , "Topical item" , "Topic; melody" , "Main idea or topic" .)