Cool self-assurance (6)
I believe the answer is:
aplomb
'cool self-assurance' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)