Notice red shift is hidden (9)
I believe the answer is:
recondite
'is hidden' is the definition.
(thesaurus)
'notice red shift' is the wordplay.
'shift' indicates anagramming the letters.
'notice'+'red'='noticered'
'noticered' is an anagram of 'RECONDITE'.
(Other definitions for recondite that I've seen before include "Mysterious" , "(Of knowledge) obscure" , "Abstruse" , "Direct one to be obscure or difficult" , "Difficult to understand" .)