The idiotic teen clarified it (8,7)
I believe the answer is:
definite article
'the' is the definition.
('the' is the definite article in English)
'idiotic teen clarified it' is the wordplay.
'idiotic' is an anagram indicator.
'teen'+'clarified'+'it'='teenclarifiedit'
'teenclarifiedit' anagrammed gives 'DEFINITE ARTICLE'.
(Other definitions for definite article that I've seen before include "Black and white feature in paper" , "In English, it's always the" , "Maybe the" , "part of speech" .)