Cosmic rays primarily penetrating new atlas (6)
I believe the answer is:
astral
'cosmic' is the definition.
(astral means relating to stars)
'rays primarily penetrating new atlas' is the wordplay.
'primarily' suggests taking the first letters.
'penetrating' means one lot of letters goes inside another.
'new' indicates anagramming the letters.
The initial letter of 'rays' is 'r'.
'atlas' with letters rearranged gives 'astal'.
'r' going inside 'astal' is 'ASTRAL'.
(Other definitions for astral that I've seen before include "Pertaining to the stars" , "Characteristic of stars" , "Like the stars" , "Relating to stars" , "Universal" .)