Junior fool put on new satin shirt (9)
I believe the answer is:
assistant
'junior' is the definition.
(thesaurus)
'fool put on new satin shirt' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'put on' says to put letters next to each other.
'new' is an anagram indicator.
'shirt' becomes 't' (as in a T-shirt).
'satin' with letters rearranged gives 'istan'.
'ass'+'istan'+'t'='ASSISTANT'
(Other definitions for assistant that I've seen before include "one helps out" , "Deputy, subordinate" , "One helping" , "second" , "help required?" .)