Perseus, for one (13)
I believe the answer is:
constellation
'perseus for one' is the definition.
(perseus is a kind of constellation)
This is the entire clue.
(Other definitions for constellation that I've seen before include "Group of stars" , "E.g Orion, Andromeda" , "Twins, perhaps" , "Cassiopeia, for example" , "Formation of stars" .)