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 subsidiary indication.
'shift' indicates an anagram.
'noticered' anagrammed gives '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".)