It's unknowns supporting Radiohead — splendid! (5)
I believe the answer is:
ritzy
'splendid' is the definition.
(thesaurus)
'it's unknowns supporting radiohead' is the wordplay.
'unknowns' becomes 'zy' (I am not sure about this - if you are sure you should believe this answer much more).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'radiohead' becomes 'r' (head letter of 'radio').
'it'+'zy'='itzy'
'itzy' put after 'r' is 'RITZY'.
Can you help me to learn more?
(Other definitions for ritzy that I've seen before include "Expensively stylish" , "Ostentatiously stylish" , "Richly elegant" , "Luxuriously elegant" , "Ostentatiously opulent (sl.)" .)