Start to judge an unlikely teen, a US author of classics (4,6)
I believe the answer is:
jane austen
'author of classics' is the definition.
(author of classic novels)
'start to judge an unlikely teen a us' is the wordplay.
'start to' suggests taking the first letters.
'unlikely' indicates an anagram.
The first letter of 'judge' is 'j'.
'teen'+'a'+'us'='teenaus'
'teenaus' anagrammed gives 'eausten'.
'j'+'an'+'eausten'='JANE AUSTEN'
(Other definitions for jane austen that I've seen before include "English novelist, d. 1817" , "English Novelist - seen a junta (anag)" .)