Cool self-assurance (6)
I believe the answer is:
aplomb
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cool self-assurance</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for aplomb that I've seen before include "Dignified self-confidence" , "Nonchalant confidence" , "'Poise, sang-froid (6)'" , "Self-satisfied dignity, complacency" , "Self-satisfied poise" .)