Leader includes awful rant in speech, only with total sincerity (5,3,4)
I believe the answer is:
heart and soul
'with total sincerity' is the definition.
'leader includes awful rant in speech only' is the wordplay.
'leader' becomes 'head' (head is a kind of leader**).
'includes' indicates putting letters inside.
'awful' is an anagram indicator.
'in speech' shows a homophone (sound like).
'only' becomes 'sole' ('sole' can be a synonym of 'only').
'sole' is a homophone of 'soul'.
'rant' is an anagram of 'rtan'.
'head' enclosing 'rtan' is 'heartand'.
'heartand'+'soul'='HEART AND SOUL'
(Other definitions for heart and soul that I've seen before include "With complete devotion" , "With complete sincerity" .)