Sirius (7)
'sirius' is the definition.
(Sirius is the Dog Star)
This is the entire clue.
(Other definitions for dog star that I've seen before include "Night light" , "Sirius in the sky" , "slightly flashy night-spot?" , "Lassie, perhaps" , "Drags to find other name for Sirius" .)
(Other definitions for dogstar that I've seen before include "animal on TV?" , "This the brightest" , "Bright night light" .)