Awful vanity punctured by English simplicity (7)
I believe the answer is:
naivety
'simplicity' is the definition.
(thesaurus)
'awful vanity punctured by english' is the wordplay.
'awful' is an anagram indicator.
'punctured by' is an insertion indicator (I've seen this in other clues).
'english' becomes 'e' (abbreviation).
'vanity' anagrammed gives 'naivty'.
'naivty' going around 'e' is 'NAIVETY'.
(Other definitions for naivety that I've seen before include "Lack of experience or maturity" , "Ingenuousness" , "Quality of excessive simplicity or innocence" , "Navy tie shows lack of sophistication" , "Quality of impractical innocence" .)