The brightest star in the night sky (7)
(Other definitions for dog star that I've seen before include "'Another name for the Sirius constellation (3,4)'" , "What may not be seen on cloudy night" , "Sirius in the sky" , "slightly flashy night-spot?" , "Drags to find other name for Sirius" .)
(Other definitions for dogstar that I've seen before include "Bright night light" , "animal on TV?" , "This the brightest" .)