The secret of self-assurance (10)
I believe the answer is:
confidence
I believe this is a double definition.
'the secret' is the first definition.
(confidence is a kind of secret)
'self-assurance' is the second definition.
('confidence' can be a synonym of 'self-assurance')
'of' acts as a link.
(Other definitions for confidence that I've seen before include "Shared secret - belief" , "in which one's relations will not be mentioned" , "Trust" , "belief in one's own abilities" , "courage" .)