Planet or dog star (5)
I believe the answer is:
pluto
I believe this is a double definition.
'planet' is the first definition.
(Pluto is an example)
'dog star' is the second definition.
(I've seen this before)
'or' acts as a link.
(Other definitions for pluto that I've seen before include "Dwarf planet" , "wartime acronym" , "Hellish leader" , "Planet beyond Neptune" , "Greek god of the underworld" .)