Silly yet vain innocence (7)
I believe the answer is:
naivety
'innocence' is the definition.
(naive can mean innocent or gullible)
'silly yet vain' is the wordplay.
'silly' indicates anagramming the letters.
'yet'+'vain'='yetvain'
'yetvain' is an anagram of 'NAIVETY'.
(Other definitions for naivety that I've seen before include "artlessness" , "Quality of excessive simplicity or innocence" , "Navy tie shows lack of sophistication" , "Lack of sophistication or worldliness" , "credulousness" .)