Confidence in one's own abilities (4-6)
'confidence in one's own abilities' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for self-assurance that I've seen before include "Confidence in one's own abilities" , "Confidence, assertiveness" .)
(Another definition for self-esteem that I've seen is " Confidence in one's own abilities and worth".)