Collections of star systems (8)
I believe the answer is:
galaxies
'collections of star systems' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for galaxies that I've seen before include "Collections of star systems" , "Groups of stars such as the Milky Way" , "Large independent systems of stars" , "Independent systems of stars in space" , "Lots of stars" .)