Notice red shift is hidden (9)
I believe the answer is:
Here is my best explanation:
'is hidden' is the definition.
'notice red shift' is the wordplay.
'shift' indicates anagramming the letters.
'noticered' is an anagram of 'RECONDITE'.
(Other definitions for recondite that I've seen before include "Enigmatic", "(Of knowledge) obscure", "Obscure", "Direct one to be obscure or difficult", "Difficult to understand", "not widely known", "Mysterious" and "Abstruse".)