Yours truly has spoken of self-esteem (6)
I believe the answer is:
morale
'self-esteem' is the definition.
(thesaurus)
'yours truly has spoken' is the wordplay.
'yours truly' becomes 'me' (the person who set this crossword).
'has' is an insertion indicator.
'spoken' becomes 'oral' (similar in meaning).
'me' enclosing 'oral' is 'MORALE'.
'of' acts as a link.
(Other definitions for morale that I've seen before include "Degree of optimism (of troops, perhaps)" , "State of a group's confidence and spirit" , "Spirits" , "Mood" , "Feeling of well-being" .)