Shabby shifts worn by one juvenile (7)
I believe the answer is:
babyish
'juvenile' is the definition.
(thesaurus)
'shabby shifts worn by one' is the wordplay.
'shifts' indicates an anagram.
'worn by' means one lot of letters goes inside another (outside letters are worn by those on the inside).
'one' becomes 'i' (Roman numeral).
'shabby' with letters rearranged gives 'babysh'.
'babysh' placed around 'i' is 'BABYISH'.
(Other definitions for babyish that I've seen before include "Typical of new arrival" , "Infantile" , "Immature" .)