Insecure at heart — still happy (8)
I believe the answer is:
ecstatic
'happy' is the definition.
(thesaurus)
'insecure at heart still' is the wordplay.
'at heart' means to look at the middle letters.
'still' becomes 'static' (synonyms).
The centre of 'insecure' is 'ec'.
'ec'+'static'='ECSTATIC'
(Other definitions for ecstatic that I've seen before include "Showing great delight" , "on top of the world" , "Feeling immense joy" , "elated" , "Overjoyed, over the moon" .)